Since January 1st, 2026, a new diagnostic must be conducted in Montpellier to complete a real estate sale: the wastewater diagnosis.

The Water Authority of Montpellier Méditerranée Métropole has clarified the practical procedures, which we summarize here and provide our advice on, as the timeframes remain significant.

What is the purpose of the wastewater diagnosis?

The diagnosis has a single objective: to verify the compliance of private collective sanitation installations during a real estate sale.

Concretely, it involves checking that:

All wastewater is properly collected and directed to the public network.
There is no rainwater discharge into the wastewater network.
The internal network shows no stagnation or anomalies (old septic tank, breaks, defective access point, etc.).
Anti-backflow devices are present if necessary.
A connection box at the property boundary exists for recent constructions.

This inspection addresses several issues highlighted by the M3M Authority:

Better understanding the condition of private assets.
Fighting pollution of natural environments.
Limiting stray clear water in networks.
Clearly informing buyers about the condition of installations.

In short: this diagnosis protects both the community… and the buyer.

Who is concerned?

The measure applies to:

Single-family homes.
Horizontal condominiums.
Multi-unit buildings (with inspection of common areas).
Extensions and annexes.

For vertical condominiums, the seller of a unit is exempted from producing an individual certificate, provided that an inspection report of common areas exists.

The text is based in particular on article L1331-4 of the Public Health Code, which reminds us that private wastewater conveyance structures are the exclusive responsibility of the property owner.

How is the inspection conducted?

The procedure is now clearly defined.

The request is made:

Via an online form on the Authority's website.
By email to the dedicated address.
By phone via a specific number.

The process follows several steps:

Property owner's request.
Advance payment for the inspection.
Appointment scheduling.
Site visit completion.
Report drafting and transmission.

For single-family homes, inspections are carried out directly by Authority staff.

For condominiums, they may be performed by a service provider under the Authority's supervision in coordination with your condominium association.

What is the validity period?

The report is valid for 10 years, provided no modifications to the installations have been made in the meantime.

In case of non-compliance:

Work is mandatory within 6 months to 1 year.
A surcharge on the sanitation fee can be up to 400% in case of delay.

This point is far from trivial. It can have significant financial impact for the seller… or for the buyer if nothing has been anticipated.

What does the diagnosis cost?

Inspections are billed at a flat rate depending on the type of property (single-family home, multi-unit building, commercial space, etc.).

Final rates are subject to validation by the Authority's Board of Directors. The exact cost therefore depends on the nature of the property and its configuration.

This expense must be factored into the sale budget, just like other mandatory diagnostics.

Early 2026: approximately 2 months timeframe

This is probably the most important information to retain. In early 2026, the average timeframe for obtaining an appointment and then the report is approximately two months.

This means that if you sign a preliminary agreement without having anticipated the inspection, you run a real risk of delaying the final signature.

Our recommendation is therefore simple: anticipate from the start of property marketing, just like with other diagnostics.

Collective diagnosis and SPANC: be careful not to confuse

The mandatory inspection in Montpellier concerns collective sanitation, meaning properties connected to the public sewer system.

Conversely, for properties with autonomous sanitation (outside the collective network), it is SPANC that intervenes as part of a specific diagnosis, with its own rules and timeframes.

The issues are different, but the objective remains the same: secure the transaction and inform the buyer.