What sellers, co-ownership buildings, and future buyers need to know

As of January 1, 2026, a new document becomes essential for selling a property in Montpellier and throughout the Metropolitan area: the wastewater diagnosis. This requirement applies to both single-family homes and co-ownership buildings and aims to verify compliance of collective sewage system installations.

This diagnosis is conducted by the Water Authority and now complements the technical sales file, alongside electricity, lead, and asbestos reports.

Essential point:

Agreements signed before January 1, 2026 are not affected, even if the notarial deed is executed in 2026.
Confirmed in the official letter dated December 2, 2025 from the Water Authority of Montpellier Mediterranean Metropolis to the notary association, which we have attached.

Why does this diagnosis become mandatory?

The objective is twofold:

Verify that each property is properly connected to the collective sewage system.
Identify any defects or non-compliant discharges, often invisible to a homeowner.

This check prevents major anomalies (rainwater sent into wastewater, improper connections, illegal discharges) that can disrupt the public network.

The diagnosis does not prevent the sale: it must simply clearly inform the buyer.

What the official letter says

No diagnosis required:

For sales where the agreement is signed before January 1, 2026.
For notarial deeds resulting from an agreement signed in 2025, even if the deed is signed in 2026.

Mandatory diagnosis:

For all sales where the agreement is signed as of January 1, 2026.
For units in co-ownership buildings if the preliminary contract is signed after December 31, 2025.

The intervention timeframe announced by the Water Authority: maximum 15 business days after the request.

Source: official letter from the Water Authority Montpellier Mediterranean Metropolis (12/02/2025).

Diagnosis cost

The fee is €180 for a single-family home.
For apartment buildings, the price is not yet known to our services.

How to request the diagnosis?

The request is made to the Water Authority of Montpellier Mediterranean Metropolis – Operations Department:

Email: controleeu@regiedeseaux3m.fr
Phone: 07 64 46 31 20

The inspection is conducted on-site by a technician assigned by the Water Authority.

Practical advice to prepare ahead

For home sellers

Request the inspection before even listing the property.
Verify that the access points are accessible.
Allow two to three weeks for peace of mind.

For co-ownership buildings

Add the implementation of the comprehensive diagnosis to the agenda of the next general assembly meeting.
Avoid multiple individual inspections, which are always more costly.
Ask the property management company to plan ahead to avoid blocking future sales.

Alpaca Immobilier's advice

Plan ahead.

As you get closer to January 2026, more requests will come in and delays are likely to increase. If you were thinking of waiting until next year for your agreement, it might be better to sign this year to avoid this somewhat... unnecessary expense!

For co-ownership buildings, a comprehensive inspection is not only more logical but also much more economical than repeated individual inspections with each sale.

Source:

2025-12-02 letter sewage system inspection requirement Download