Hydrogeological constraint in Italy: how to check it, who authorises and what building requires

The hydrogeological constraint is an Italian land protection dating from 1923 that concerns the stability of the ground, not its appearance: in constrained areas, earthworks and changes to the land need an authorisation, because careless works can trigger landslides, erosion or a change in how water drains downhill.

For anyone buying a plot on a hillside or in the foothills, or planning a villa with excavations, cut-and-fill, retaining walls, a pool or a basement cellar, it is one of the first checks to make. It can add a consent to the building-permit route and conditions to the site, and it is far better discovered before signing the preliminary sale contract than once the excavator is on the plot.

This guide explains what the rule says, how the hydrogeological constraint differs from the landscape constraint and from the hazard maps of the Italian basin plans, how to check whether a plot is affected, who authorises in the Veneto region, which documents are filed and what changes for excavations, retaining walls, pools and cellars.

It is written by Federico Cappellina Architects, an architecture practice in Verona that designs houses in north-east Italy and Lombardy and supervises their construction, for foreign owners and buyers who are building or renovating in Italy. Sources are linked in their current or official text as we read it; what comes from our own practice is stated as such. This page is not legal advice or a geological report: rules vary by region and municipality and must be checked for the specific plot with the municipality and the competent office.

The hydrogeological constraint in brief

The hydrogeological constraint (vincolo idrogeologico) comes from Article 1 of Royal Decree 3267 of 1923 and covers land of any nature and use that, through uses contrary to the rules, risks losing its cover, its stability or its water regime.
It is not the landscape constraint (Legislative Decree 42/2004) nor the geological hazard classification of the district basin plans: these are three separate tools with different consents.
In Veneto, any work involving earthmoving in an area under the hydrogeological constraint needs the authorisation of the forestry authority (Article 36 of the regional forestry rules).
For earthworks tied to building projects and their directly connected infrastructure, the authorisation is issued by the municipality; for other works, the regional forestry offices remain responsible.
The regulation lists a few minor works exempt from authorisation, including small stabilising structures lower than 1.5 metres and fences.
The check should be made before the plot is bought and before the design is fixed: afterwards, the room to adjust the project is smaller.

Hydrogeological constraint and the building site: AV Home in Dueville seen from the street with the cantilevered volume under construction, a project by the studio — AV Home, Dueville (Vicenza): a project by the studio, shown as a reference of language, not as an example of a plot under the hydrogeological constraint.

What the hydrogeological constraint is

Article 1 of Royal Decree 30 December 1923, no. 3267 opens the national law on the reorganisation of woodland and mountain land. It places under a constraint for hydrogeological purposes land of any nature and use that, through uses contrary to the rules of Articles 7, 8 and 9, may, to the public’s detriment, lose its cover, lose stability or disturb the water regime.

The text we read on Normattiva, the official database of Italian legislation, is in force from 1 June 1924, and the page indicates the last update of the act as 1 February 2013; no amendments are shown for Article 1.

Three expressions in the definition deserve attention. The first is «land of any nature and use»: the hydrogeological constraint is not limited to forests, although it was created to protect them; it can burden meadows, pastures, farmland, gardens and the grounds of buildings. The second is «public detriment»: the interest protected is collective, the stability of slopes and the regulation of water downstream, not the value of a single plot. The third is «may»: the constraint is preventive, applying where the risk is plausible rather than where damage has already occurred.

What Articles 7, 8 and 9 say

The articles called up by the first one make the constraint concrete. Article 7 provides that, on constrained land, converting woodland to another type of land use and turning untilled land (terreni saldi) into regularly worked land are subject to authorisation, on conditions set case by case to prevent the damage of Article 1.

Article 8 gives the competent authority the task of setting, only where necessary and even temporarily, the rules for managing woods and pastures, removing protective shrubs and working the soil. Article 9 concerns grazing on constrained land. The decree refers to a «Forestry Committee», a 1923 body: today authorisations are issued by the authorities identified in regional law, which for Veneto we describe below.

The hydrogeological constraint is not a ban on building

We say this at once because it is the most frequent misunderstanding. From the texts we read, the hydrogeological constraint does not declare a plot unbuildable: it makes earthworks and changes to the soil subject to an authorisation.

That is our reading, an inference: any ban on building in an area, where there is one, derives from other instruments, such as the municipal planning scheme, landscape rules or geological hazard maps. For this reason checking the constraint is never enough on its own, and this guide and our guide to buying land in Italy should be read together.

Hydrogeological constraint, landscape constraint and basin plans: three different things

Anyone researching the hydrogeological constraint will meet two other protections with similar names. Telling them apart avoids wrong applications and surprises during the review.

Hydrogeological constraint (Royal Decree 3267/1923): protects the stability of the ground and the water regime; the authorisation concerns earthmoving and changes to the soil. In Veneto it is governed by the regional forestry law and by the forestry rules (Prescrizioni di massima e di polizia forestale).
Landscape constraint (Legislative Decree 42/2004): protects the look of the landscape. Article 142 lists the areas protected by law, among them a 150-metre strip from the banks of rivers, streams and watercourses entered in the statutory lists, and land covered by forests and woods. The authorisation is a different one, issued under the landscape rules, and follows a different route; it is not covered in this guide.
Hazard and risk in the basin plans (PAI): maps and rules of the river basin district authority. According to the page of the Eastern Alps District, the PAI is a part of the basin plan and today concerns landslide hazard and geomorphological instability; hydraulic hazard has moved to the Flood Risk Management Plan, approved by a prime ministerial decree of 1 December 2022. A plot can be in a hazard class without being under the hydrogeological constraint, and the other way round.

The three protections can overlap. A wooded area, for example, may be under the hydrogeological constraint and, as woodland, also under the landscape constraint. The Veneto regulation recognises this in one precise case: for converting woodland to another land use, the forestry authorisation also counts for Article 146 of Legislative Decree 42/2004, the landscape authorisation. In other cases the procedures are separate and the designer has to coordinate them. Our guide to the urban planning certificate explains how to read the planning side of a plot.

How to check whether a plot is under the hydrogeological constraint

There is no single register that says, with the value of proof, whether a land parcel is constrained. In our practice the check on the hydrogeological constraint proceeds in layers, from the general to the precise, and always ends with a written answer from the municipality.

Cadastral details of the plot: sheet and parcel numbers, from the cadastral extract and map. Without them no office can answer precisely.
A written request to the municipality (building or planning office) asking whether the parcels fall in an area under the hydrogeological constraint. This is the check that counts, because in Veneto the municipality issues the authorisation for building-related earthworks.
The constraints map of the municipal plan: in our practice, in Veneto, the constraints sheets of the municipal territorial plan show the protected areas. We check on each municipality that the sheet is up to date.
The urban planning certificate: it says how the plan classifies the land. Whether it also mentions the hydrogeological constraint depends on the municipality; we always ask in writing for it to be stated.
Regional territorial data: the Region publishes maps on its geoportal; they are useful as a first indication, but we could not verify today the update date of the layer on the constraint, so an online map does not replace the municipality’s answer.
Regional forestry offices: for doubtful cases, or when the plot has woodland, one contacts the office for the province. The Region’s page states that the offices deal, among other things, with checks on the presence of woodland and with earthworks in areas under the hydrogeological constraint.
A technical site visit: slope, embankments, woodland or trees, ditches and gullies, dry-stone walls, signs of slippage. The constraint is on paper, but the risk is made of ground.

The sources do not give a response time for these checks; they are requests to be made before the preliminary contract, with margin. A plot that is buildable on paper but has significant embankments deserves a preliminary note from a geologist before the price is agreed.

Who authorises the hydrogeological constraint in Veneto

The national rule dates from 1923, but the administration of the constraint is regional. The text we read for Veneto is Regional Regulation 7 February 2020, no. 2, which contains the forestry rules (Prescrizioni di massima e di polizia forestale) adopted under the regional forestry law (Regional Law 52/1978), in the 2022 coordinated version published by the Region. The regulation may have been amended since: the current text must be checked before filing.

The municipality for building earthworks

Article 37, paragraph 2, provides that for works involving earthmoving for building projects and the infrastructure strictly connected with them, the authorisation of Article 36 is issued by the competent municipality, under the delegation of administrative functions in regional law 58/1994.

The regional form we read explains that this delegation applies to building works and strictly connected works, such as buildings of any kind, service connections, the arrangement of grounds, access roads and parking. For a villa, therefore, the application for excavations and foundation digging is generally filed with the municipality, often together with the building application.

The Region for other works

According to the same form, the Forestry Directorate remains competent for earthmoving for agricultural improvements (for example grubbing up and replanting vineyards, infill, levelling), environmental restoration, stabilisation of embankments and unstable land, agro-forestry tracks and roads, utility networks, temporary deposits outside building sites and geological investigations with boreholes.

The Region’s page on the hydrogeological constraint also lists reductions of wooded area and restoration after unlawful clearing. The offices are organised in five structures: Belluno, Padua-Rovigo, Treviso-Venice, Verona and Vicenza, with forms for each. The same page gives a single certified-email address and the office addresses, and should be consulted for up-to-date details.

A practical consequence: the garden of a villa with major levelling or infill, when it is not part of a building project, may depend on the regional offices and not on the municipality. It is one of the cases where it is worth asking the office for written confirmation before moving earth.

Other regions

The 1923 rule is national, but competences and procedures change from region to region. What is said above applies to Veneto and to the sources we read. For a plot in Lombardy or elsewhere the authorities and forms are different and we do not cover them here: the check must be made with the municipality and with the forestry office of the region concerned.

When authorisation is needed: earthworks

Article 36 of the regulation is the core of the system. It provides that the reduction of woodland, the transformation of untilled land into regularly worked land and any other work involving earthmoving in an area under the hydrogeological constraint are subject to authorisation by the forestry authority.

Article 4 defines «earthworks» (movimenti terra) as works that change the layout of the ground, including the permanent change of use of constrained land. In our view this therefore covers foundation excavations, cut-and-fill, embankments, levelling, ramps and new access roads: any work that changes the levels and the profile of the ground.

The application may also be filed by someone who is not the owner, with the owner’s written consent.
The provisions do not apply to routine maintenance.
For the conversion of woodland to another land use the forestry authority carries out the review after hearing the regional decentralised technical commission, and the outcome is published for fifteen consecutive days on the municipal notice board.
The provisions also apply to woods not under the hydrogeological constraint where woodland is reduced. Unless a sum is paid to the regional fund, the application identifies the land where compensation works will be carried out, preferably within the same river basin.

Works exempt from authorisation

Article 37 bis lists minor works that do not alter the hydrogeological layout and are exempt from authorisation. In the coordinated text we read, those relevant to someone with a house and garden are:

building, maintaining, replacing and adapting fences, gates, signs and other signage, and furnishings;
building and rebuilding small stabilising structures of the ground less than 1.5 metres high, mainly using soil bioengineering techniques, dry-stone walls and small works for draining rainwater using natural materials;
earthmoving for building and maintaining utility networks along existing paved or stabilised roads;
converting meadows and pastures into vegetable plots by surface working (maximum depth 30 centimetres, average slope below 30 per cent, up to 2,000 square metres) and improving meadows within similar limits.

The exemption concerns the forestry authorisation, not the building title: a fence or a small wall may still require a building notice or other consents. And the list may have been updated after the 2022 coordinated text we read.

Hydrogeological constraint and house design: CD Home in Verona, a single-storey villa on a lawn, a project by the studio — CD Home, Verona: a project by the studio, shown as a reference of language, not as an example of a plot under the hydrogeological constraint.

Documents for the authorisation and the geological report

Every office has its own forms, and for applications filed with the municipality the municipality’s forms apply. The regional earthworks form of the Verona office, which we read, is nonetheless a good index of what is asked. The application is signed by the owner, the tenant (with a consent deed) or an appointed professional, states the works, the cadastral data, the surface concerned and the current land use, and attaches:

two revenue stamps of 16 euros, one for the request and one for the authorisation, or proof of electronic payment;
a map setting of the site;
an extract of the cadastral map at 1:2,000 showing the area of works;
photographs;

and, depending on the scale of the works and signed by a qualified professional:

a detailed technical report;
detailed drawings: plan and sections of the existing and proposed state, location map, longitudinal profile;
a geological and geotechnical report;
photographs with the viewpoints marked on the plan;
.shp or .gpx files of the areas, by link.

The geological report in practice

The geological and geotechnical report is the document that turns «earthmoving» into numbers: soil layers, groundwater, stability of excavation faces, behaviour of the ground under load. In our practice, for a sloping plot, a geologist is commissioned before the preliminary design is finalised, not after: the conclusions guide the position of the building, the ground-floor level and the type of foundations. In some cases municipal plans or basin rules also ask for a compatibility report; this is not a general requirement and must be checked on the municipal plan.

Timing, costs and penalties

The sources we read give no deadline for concluding the authorisation procedure for earthworks: the only period that appears is the fifteen days’ publication on the municipal notice board for converting woodland. We therefore give no indicative timing: it has to be asked of the office, and the programme should leave a margin.

The direct costs the form indicates are the revenue stamps; professional fees and any guarantees vary. As for penalties, Article 36 refers to the administrative sanctions of Law 950/1967 and to Articles 24 and 26 of Royal Decree 3267/1923, and Article 41 allows the authority to order restoration works, even carrying them out at the offender’s expense. We do not quote amounts because we have not read them.

The hydrogeological constraint and the building permit

The hydrogeological constraint neither replaces the building title nor is replaced by it: they are two consents, normally coordinated within the same review. How they are coordinated depends on the municipality: in some cases the forestry authorisation must be attached to the application, in others it is obtained by the office during the review.

We have not read a uniform rule, and for this reason the first step is to ask the municipality’s one-stop building office how it handles the file. Our guide to the building permit in Italy describes the general procedure; the one on CILA and SCIA covers titles for minor works.

The principle we follow is simple: the project is set up knowing that the area is constrained, with excavation sections and levels that minimise earthmoving, and the papers for the forestry authorisation come from the same drawings as the permit, not from a second version. In a well-run application the two authorisations tell the same project.

The hydrogeological constraint and buying land

When buying land in Italy, the hydrogeological constraint is checked before signing, because it affects cost and feasibility. In our practice, for a plot in hills or foothills, before the preliminary contract we ask for:

the municipality’s written answer on the hydrogeological constraint and on other constraints (landscape, hazard maps);
the urban planning certificate, asking that constraints be stated;
a first assessment of the ground: slope, woodland, embankments, water;
an estimate of the excavation required for the house you want to build, because the cost of earthworks and retaining structures can change whether the purchase is worthwhile;
an indication of any woodland to be reduced, with compensation or payment to the regional fund, when the house requires trees to be removed.

If the check takes time, the agreement with the seller may make the preliminary contract depend on the outcome of the authorisations: how to word the clause must be settled with the notary. The building volume calculation and the hydrogeological constraint should be read together: a plot may have the volume on paper and excavations that cannot be carried out without costly works.

Cut-and-fill and retaining walls

Excavation is the earthwork par excellence and, in a constrained area, the first one the office looks at. The questions we ask in design are always the same: how much ground is moved, where does the material go, how high is the excavation face, how is it supported, where does the water go.

Building-footprint excavation: the width of the cut and the height of the faces determine the technique (sloped cut, retaining piles, micro-piles) and the geological report; an architectural choice such as a house partly sunk into the slope is paid for in excavation and retaining works.
Fills and embankments: these are earthworks too, with compaction and drainage problems.
Retaining walls: the regulation exempts small stabilising structures lower than 1.5 metres built mainly with soil bioengineering, and dry-stone walls. A structural reinforced-concrete wall does not fall within the description we read: we treat it as a case to check with the office, and in any case it needs structural calculation and a building title.
Excavated soil: surplus material and temporary deposits have their own rules, including environmental ones, which we do not cover here; the regional form places temporary deposits outside building sites among the competences of the regional offices.

Excavation for pools and cellars

A buried pool and a cellar are excavations, and in a constrained area they are earthworks like any other. In our practice two situations are distinguished.

Pool: if it is part of the grounds of a villa and tied to the building project, it falls within the municipality’s competence for appurtenances; the design stage must provide for the excavation, the balance of soil, the drainage of the basin and the discharge of emptying water, which cannot be channelled onto the slope. The regulation requires (Article 39) that surface run-off from works in a constrained area be managed so as not to damage the slopes below.
Cellar or basement room: excavation against sloping ground involves earth pressure, groundwater or slope water and waterproofing. The geological report indicates whether perimeter drainage is needed and where to release the water. Our guide to the wine cellar in a villa deals with the layout side.

In both cases the decision to excavate, and how much, is taken with the geologist before the elevation is drawn: moving the pool uphill or downhill of a wall, or the cellar toward the slope rather than the road, changes the retaining works and therefore the cost.

During construction: conditions, guarantees and restoration

The authorisation does not close the subject: the regulation sets rules for execution that the site supervision must enforce.

Building works with earthmoving must minimise the cutting of trees and respect the conditions imposed; all excavated and filled surfaces must be arranged, consolidated and re-grassed (Article 37, paragraph 3).
Surface run-off from works must be managed so as not to damage the slopes below (Article 39).
The forestry authority may require surety or deposit guarantees for proper execution, and releases them after verifying proper execution and the functioning of the works (Article 40).
In case of breaches, the authority, or the municipality for building projects, may order restoration, consolidation or adaptation; if offenders do not comply, it acts at their expense by keeping the deposits or calling on the guarantees (Article 41).

On site this means three things: a plan for the rainy season, because excavations left open in autumn are the most vulnerable; a place for materials and soil that does not weigh on the slope; and a site record documenting each phase, because the release of the guarantees depends on the authority’s findings. Where the site has several contractors, safety duties also apply: our guide to construction site safety in Italy summarises them.

How our practice sets up a project in a constrained area

This is our practice, not a legal procedure. For a house in hilly terrain or with embankments the sequence is as follows.

Before the design engagement, we check the hydrogeological constraint with the municipality and visit the site, to tell the client which consents to expect.
With the client, we decide how much excavation to accept: the shape of the house grows from the ground, not against it.
With the geologist, we set the investigation and read the results before the preliminary design.
In the design, sections show the natural and the proposed profile, with the balance of excavated and filled volumes, the retaining works and the management of water.
At filing, the building permit and the authorisation for earthworks are coordinated, with the same drawings.
In the specification and bill of quantities, excavation, retaining and drainage items are separate and measured, as in the bill of quantities.
On site, supervision checks levels, excavation faces and water disposal at each phase, and documents what is needed to release the guarantees.

Hydrogeological constraint and design details: perforated screen on the facade of SD Home in Soave lit at night, a project by the studio — SD Home, Soave (Verona): a detail of a project by the studio, shown as a reference of language, not as an example of works in a constrained area.

Frequent mistakes about the hydrogeological constraint

Confusing it with the landscape constraint: they are different authorisations, with different offices and procedures, and a plot may have one, the other or both.
Thinking it concerns only woodland: the rule speaks of land of any nature and use.
Believing it bans building: in the texts we read it is an authorisation for earthworks; bans come from other instruments.
Relying on an online map: cartography is an indication; the valid answer is the municipality’s written one.
Buying the plot before checking: when the check reveals embankments or woodland, the agreed price does not account for excavation and retaining costs.
Opening the site without the authorisation: this exposes the owner to restoration at the offender’s expense and to penalties.
Moving earth in the garden at the end of the works to level, infill or relocate surplus without checking whether an authorisation is needed: levelling of land may fall under the Region.
Neglecting water: damage often comes from badly directed water, not from the excavation itself.

Italian terms at a glance

Vincolo idrogeologico: the hydrogeological constraint of Royal Decree 3267/1923.
Movimenti terra: earthworks, works that change the layout of the ground.
Autorità forestale: the forestry authority, in Veneto the regional forestry structure for the territory.
Prescrizioni di massima e di polizia forestale (PMPF): the regional forestry rules, with the articles on authorisation.
PAI (Piano per l’Assetto Idrogeologico): the basin plan on landslide and geomorphological hazard.
Perizia geologico-geotecnica: the geological and geotechnical report on the ground.
Certificato di destinazione urbanistica (CDU): the urban planning certificate of a plot.
Sportello unico per l’edilizia (SUE): the municipal one-stop building office.

The hydrogeological constraint: frequently asked questions

What is the hydrogeological constraint in Italy?

It is the constraint of Royal Decree 3267/1923 on land of any nature and use that, through uses contrary to the rules, may lose its cover, lose stability or disturb the water regime. In constrained areas, earthworks and changes to the soil need an authorisation.

Does the hydrogeological constraint prevent building?

In the texts we read it is not a ban on building: it makes earthworks subject to authorisation. Any ban on building depends on the planning scheme, landscape rules or hazard maps and must be checked case by case.

How do I know whether a plot is under the hydrogeological constraint?

By a written request to the municipality giving sheet and parcel numbers, by reading the constraints map of the municipal plan and the urban planning certificate, and, in doubtful cases, through the regional forestry offices. Online maps are an indication, not proof.

Who grants the authorisation for the hydrogeological constraint in Veneto?

For earthworks tied to building projects and their directly connected infrastructure, the municipality; for other works, such as levelling, agricultural arrangements or agro-forestry roads, the regional forestry offices (Articles 36 and 37 of the forestry rules).

Which documents are needed for the authorisation?

The regional form we read lists revenue stamps, a map setting, a 1:2,000 cadastral extract, photographs and, for larger works signed by a qualified professional, a technical report, drawings and a geological and geotechnical report. The municipality may have its own forms.

Do a pool or a basement cellar need the authorisation?

In a constrained area they are excavations, so earthworks, and fall within the authorisation unless they are routine maintenance. If they are part of a building project, the municipality is generally competent. Confirmation should be asked of the office.

Is a retaining wall exempt from the authorisation?

The regulation exempts small stabilising structures lower than 1.5 metres, mainly soil bioengineering, and dry-stone walls. A structural reinforced-concrete wall does not fall within the description we read: check with the office, and it needs a building title and calculation in any case.

Hydrogeological constraint: checklist before you buy or design

Before buying a plot, or starting a project with excavations, cuts or retaining walls, the hydrogeological constraint deserves the checks below.

The hydrogeological constraint is verified with a written request to the municipality and to the competent regional forestry service, not by reading a map at a glance.
The hydrogeological constraint applies to land, not to the building: the check must be made on the exact cadastral parcels of the plot, including those of the access.
The hydrogeological constraint must be kept apart, in writing, from the landscape constraint and from the hazard shown by the hydrogeological plan (PAI): these are three different searches.
With the hydrogeological constraint in mind, establish whether the work counts as earthworks that need authorisation or falls among the exemptions of the regional regulation in force.
Where the hydrogeological constraint applies, excavation and fill quantities, levels and ground sections are quantified in the design before the application.
The hydrogeological constraint calls for timing coordination: the forestry authorisation and the building title do not always close at the same moment.
The hydrogeological constraint can affect pools, basement cellars, ramps and retaining walls: any work involving earthmoving must be declared.
In a preliminary purchase contract, the presence of a hydrogeological constraint deserves a clause to discuss with the notary.
A hydrogeological constraint is not read as an automatic ban on building: the outcome depends on the review by the competent authority.
The outcome of the hydrogeological constraint check is kept in the project file, with date and source.
A hydrogeological constraint check that cites the parcel numbers and the office consulted can be repeated by the next owner or designer.
The hydrogeological constraint is a question for the whole site team: the designer, the geologist and the contractor should all hold the same answer.