The hydrogeological constraint (vincolo idrogeologico): what it stops, who approves works, and what to check on an existing house

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Here is what it looks like in practice. In 2024 a Piedmont regional decision authorised works at Monticello d’Alba (Cuneo) on 1,450 m² of unwooded land under the constraint, with 2,280.43 m³ of excavation and fill: an average of 1.57 metres of earth moved over every square metre. In 2022, at Alba (hamlet of Como), a new vineyard covered 6,175.37 m² with 3,558.15 m³: 57 cm on average. Both figures come from decisions published in the regional Official Bulletin, and they show what the office looks at: how much earth you move, where, and what happens to the water afterwards.

The constraint does not ban building. It requires permission before you disturb the ground, and that permission cannot be obtained afterwards. This guide explains where it comes from, how it differs from flood-risk maps and landscape protection, who decides in Piedmont and Tuscany, and what to check on a house that already exists. It is general guidance; your plot is assessed by the competent office and a geologist.

Key terms

Three Italian constraints that sound alike

The areas do not coincide, and one approval never replaces another. A plot can sit inside the hydrogeological constraint and outside every PAI hazard zone, or the reverse. Our guide on PAI flood and landslide risk covers the second column.

Where the constraint comes from

The 1923 decree (Chapter I, Section I) placed under constraint land "of any nature and use" that, through incompatible use, could suffer erosion, lose stability or disrupt water flow, to public detriment. Its implementing regulation is Royal Decree 1126 of 16 May 1926. Article 7 requires authorisation to turn woodland into other land uses and to turn uncultivated land into regularly tilled land; regional practice extends control to any permanent change of land use.

Today the regions run it entirely (art. 61(5) of Legislative Decree 152/2006), so each region has its own law, office and thresholds. Many boundaries were drawn decades ago on maps of the time; in Tuscany, the regional map’s metadata point to a 1983 archive.

How to see it

• Zoning certificate (certificato di destinazione urbanistica, CDU): it should list the constraint. See our CDU guide.
• Regional geoportals: Piedmont publishes the boundary on its geoportal (2016 edition, scale 1:10,000); Tuscany on Geoscopio.
• Municipal constraint maps: Brescia, for instance, has a soil-protection constraints map in its town plan.

At 1:10,000 one millimetre on the map is ten metres on the ground. If the boundary runs across your plot, ask the office how it reads it before your architect starts drawing.

Who approves works

Piedmont: regional law 45/1989

Piedmont’s law (L.R. 45 of 9 August 1989, amended in 2024) splits authorisations between the Region and the towns according to the size of the works. For the provinces of Cuneo, Asti and Alessandria the regional office has been the Settore Tecnico Piemonte Sud since 1 February 2022. The documents required are set by management decision no. 368 of 7 February 2018, Annex A. The town hall will tell you which office handles your project.

Published decisions show what it costs beyond the geologist and the designer:

• a security deposit, e.g. €1,000 under art. 8(2) in decision no. 670 of 2026, paid before works start;
• a compensation or reforestation payment where required: €433.80 in decision no. 397 of 2025, and €741.80 under art. 9(3) in decision no. 233 of 13 April 2026, for demolishing and rebuilding a house at Massino Visconti (Novara).

A single-family house goes through the same process: decision no. 110 of 2 March 2026 authorised earthworks for a detached house on a plot in an approved development plan.

Tuscany: forestry law 39/2000

In Tuscany the constraint is handled under the regional forestry law (L.R. 39 of 21 March 2000) and forestry regulation 48/R of 2003. Works fall into three groups: free (arts. 98–99 of the regulation), subject to a start-of-works declaration (art. 42(7) of the law), or subject to authorisation. Towns and unions of towns share the functions, and many towns have their own regulations (Livorno, Bibbiena).

Others

In Lombardy, when woodland is converted, the office authorising the conversion also checks the hydrogeological constraint. In Veneto, regional regulation 2/2022 requires authorisation for any earth movement in the constrained area.

The swimming pool question

The project most foreign buyers have in mind for a hillside house is a pool, often with a terrace, a retaining wall and a widened access track. All three move earth. In a constrained area each needs hydrogeological clearance in addition to the building permit and, where it applies, landscape authorisation.

Ask for a rough earthworks figure before you buy. As an order of magnitude only: a 10 × 4 m pool with surrounding terrace on a slope can involve a few hundred cubic metres of cut and fill, and a retaining wall more. On a steep slope the geologist’s report will be more demanding and the deposit higher.

Cutting the cubic metres: a revised design in numbers

The figure the office looks at can be changed at design stage. An example with hypothetical dimensions, to see where the volume sits:

Dropping the basement removes 360 of the 575 m³, almost two thirds. On a 1,500 m² plot the average earth moved falls from 38 cm to 14 cm per m², well below the 1.57 m of the Monticello d’Alba case and the 0.57 m of the Alba vineyard. It does not guarantee approval, but it moves the project into the range where published decisions have said yes. A geologist can tell you before you sign which items the slope can take.

An existing house: was it authorised?

This is the check that matters most if you are buying a house already built in a constrained area. If the house, pool or access road was built after the constraint was imposed without hydrogeological authorisation, a late application does not fix it.

The Constitutional Court made this clear in judgment no. 100/2026, filed on 9 June 2026, striking down a Sardinian regional provision (L.R. 18/2025) that allowed after-the-fact regularisation of works in constrained areas. The Court said the constraint has "an essentially preventive function" and that prior authorisation is an "indispensable element". Works built before the constraint existed are a different case, addressed in judgment no. 75/2022.

So:

1. Through a records request to the town, ask for the hydrogeological authorisations for every ground work, not only the building permits: house, extensions, pool, retaining walls, road.
2. Compare their dates with the date the constraint was imposed in that area.
3. If a later work has no authorisation, treat it as something that may have to be removed, and reflect that in the price and the preliminary contract.

Unauthorised works generally are covered in our guide on abusi edilizi.

The hydrogeological constraint boundary, PAI areas, landscape constraint, slope and dated aerial photos for your plot are in the TerrenoCheck report, each with its source: €29 in one language, €39 in English and Italian. Authorisations for existing works still have to be requested from the offices. Find the plot on the map → · See a sample report

Why the building permit will not arrive by silence

Italian building permits can in some cases be granted by silence (silenzio-assenso) if the town does not decide in time. Buyers who plan around that should know that, after Law 182/2025 amended art. 20(8) of Presidential Decree 380/2001 (in force from 18 December 2025, according to the technical press), silence can form on constrained land only if the competent offices' consents, such as the hydrogeological authorisation, have already been obtained and are valid. Before the reform, case law ruled silence out altogether where the constraint applied. Either way: get the hydrogeological authorisation first, if necessary through a services conference under arts. 14 ff. of Law 241/1990. In Tuscany, town regulations such as Livorno’s require applications and start-of-works declarations to be filed online only, which in practice means through a professional with digital signature and certified email.

For a timeline, count the hydrogeological authorisation as its own step, before the building permit. If you are buying in spring hoping to swim in the pool next summer, that sequence (geologist, design, authorisation, permit, works) is the reality check: ask the office how long its recent authorisations took and plan from the answer, not from the agent’s estimate.

Step by step, from abroad

1. Boundary. Piedmont Geoportal (2016 edition, 1:10,000), Geoscopio in Tuscany, or the town plan maps elsewhere. If the line crosses the plot, ask the office in writing how it reads it.
2. Zoning certificate. The CDU should list the constraint. If it does not, but the regional map shows it, ask the town to explain in writing.
3. Which office. Town, union of towns or regional technical office: who authorises your kind of works, with which form and in what time. Italian offices reply to certified email (PEC); a local surveyor (geometra) or architect usually handles this for foreign buyers under a power of attorney.
4. Existing works. A records request for the building permits and hydrogeological authorisations of house, pool, walls and road, with dates.
5. Design and geologist. An earthworks estimate and a geologist’s first view of the slope. The geologist must be registered with the Italian professional body (Ordine dei geologi) to sign the report filed with the application.
6. Preliminary contract. If the plot’s price depends on the project, make the purchase conditional on the authorisation; if existing works lack it, reflect that in the price.

If the answer is bad

• Shrink the project. Less basement, a smaller or above-ground pool, terraces on structures instead of fill. The table above shows where the volume is.
• Move the building out of the constrained area if the boundary crosses the plot and the town plan allows it.
• Negotiate when an existing pool or wall was built after the constraint without authorisation: judgment 100/2026 says authorisation is preventive, so a later fix cannot be assumed.
• Use the conditional contract you signed. If the preliminary contract made the purchase conditional on the authorisation and the office refuses, the condition fails and the deposit (caparra) comes back. Without that clause, a refusal after signing is your loss: the seller has sold land, not a permit. Agree in the clause who files the application, by when, and what counts as a refusal (a formal refusal, or no decision by a set date), so that nobody argues about it later in a language you do not read.
• Walk away if the project that justifies the price has no realistic prospect according to the geologist. A hillside plot with a view and no pool is still a hillside plot with a view, but it should not be priced as one with a pool.

Questions to put to the seller

• Which works on the land were done after you bought it, and were any authorisations needed for them?
• Do you have the hydrogeological authorisation for the pool, the walls and the access road, or the date they were built?
• Has the town or region ever inspected the property or written about works on it?
• Has the land been affected by landslips, subsidence or water running off the slope?