You live in a new-build home delivered under VEFA (off-plan sale) contract, and sewage or musty odours persist despite several visits from the developer’s contractors. You reported the problem during the one-year defects liability period, the builder sent tradespeople round, you were told about siphons needing topping up or seals needing redoing, but the smell always comes back. This pattern points to a simple conclusion: the real source of the problem was never identified.
When repeated repairs fail to permanently eliminate the odour, it means the initial diagnosis was incomplete. What’s missing is a thorough technical investigation of the drainage system using proper instruments: food-dye tests on every fixture, cold-smoke injection into the foul and surface-water pipework, inspection of the crawl space, and hygrometry readings. This kind of investigation is the job of an independent building expert equipped and trained in drainage diagnostic protocols.
Why do the developer’s repairs so often fail?
The developer or builder usually sends back the plumber who did the original work, or a tradesperson from their own network. These call-outs aim to quickly fix what looks like the obvious cause, without a methodical diagnosis of the whole drainage system. They top up siphons, tighten seals, replace a hose \u2014 but nobody checks whether the foul water (EU) and surface water (EP) networks are properly separated, whether the soil stack is correctly vented, whether the crawl space has standing water, or whether the siphons are being emptied by a suction effect.
The fundamental problem: a symptom is treated without tracing it back to the cause. The smell disappears for a few days, then returns. The developer sometimes concludes that « everything is normal » or that « it’s down to how the house is used », when in fact the defect is structural.
An independent expert has no commercial relationship with the developer, carries out no repairs, and sells no materials. Their role is to establish the technical origin of the defect and determine liability \u2014 whether it falls under the one-year defects liability guarantee, the two-year guarantee, or the ten-year structural guarantee.
Common causes of persistent odours in new-build VEFA homes
After fifteen years in public works and site management, followed by building expertise, we find that sewage or musty odours in VEFA homes most often stem from execution defects in the drainage system or ventilation:
Siphons unfed or emptied by suction
A siphon (water trap) stops sewer gases rising back up. If a fixture (basin, shower, toilet) is rarely used, the water in the trap evaporates and lets odours through. But in VEFA homes, the most frequent cause is a suction effect: when a soil stack isn’t properly vented (missing or undersized primary or secondary venting), the flow of water creates a vacuum that empties the traps of other fixtures on the same network. This is invisible to the naked eye and requires an analysis of the stack ventilation system.
Reversed or faulty foul/surface water connections
Foul water (EU) and surface water (EP) must run through separate pipework all the way to the public sewer or the private drainage system. A reversed connection (foul water routed into the surface-water network or vice versa) or a faulty joint (missing seal, poorly fitted coupling) lets sewage leak out under the slab or into the crawl space, causing odours and damp. A dye test traces the actual path of each outlet and reveals any connection fault.
Crawl space with standing water
Where a crawl space is present, any leak in the foul-water pipework, any rainwater infiltration, or any rise in the water table can create a pocket of standing water. Stagnant water gives off a musty smell that rises into the home through service duct penetrations or gaps in the slab’s airtightness. Inspecting the crawl space with hygrometry readings and checking for water traces is essential.
Poorly vented soil stack
DTU 60.11 (the French plumbing code) requires primary ventilation (extending the soil stack above roof level) to balance pressure in the foul-water network and prevent siphons losing their seal. If this ventilation is undersized, partially blocked, or missing, odours rise back up through the sanitary fixtures even when the traps are full.
Faulty seal on buried pipework
Foul-water pipes buried under the slab or running through the crawl space can have invisible leaks (a faulty seal, a crack, a poorly made connection). These leaks let sewage escape, where it stagnates and gives off persistent odours. The ten-year structural guarantee applies where the defect compromises the soundness of the building or makes it unfit for its purpose (French building codes, Article 1792 of the Civil Code).
The independent expert’s diagnostic protocol
When odours resist the developer’s repairs, Cassini Expertise runs a complete protocol using professional measuring instruments (Trotec) and tracer tests:
Food-dye tests
Each fixture (basin, shower, sink, toilet, floor drain) receives a harmless food dye. We then check where it appears \u2014 in the foul-water manhole, the surface-water manhole, or the private drainage system. This test immediately reveals reversed connections, unseparated networks, or outlets that don’t reach their intended sewer line.
Cold-smoke injection into the pipework
A cold-smoke generator (non-toxic, no combustion) is connected to the foul-water network. The smoke, under slight pressure, travels through the pipes and escapes through any crack, faulty seal, loose fitting, or gap in the sealing. This test pinpoints invisible leaks with precision, including in buried sections or within the crawl space.
Crawl space inspection
Where access allows, the expert physically inspects the crawl space or uses an endoscopic camera to spot signs of damp, pooled water, damaged pipework, or faulty connections. Hygrometry readings complete the inspection.
Hygrometric and thermal measurements
Trotec instruments map relative air humidity, surface wall temperature, and condensation points. This data provides objective evidence of mould and points to the areas of infiltration or leakage.
Liability report
The expert report establishes the technical origin of the defect, identifies any non-compliance with French building codes and standard practice, determines which guarantee applies (one-year defects liability, two-year guarantee, or ten-year structural guarantee), and specifies the remedial work required. This document is issued under the expert’s professional liability and can be submitted to the Grasse judicial court (for Antibes and the Alpes-Maritimes) or the Toulon judicial court (for the Var).
Who pays for the expert assessment and the repairs, depending on the cause?
Liability depends on the nature of the defect and which guarantee applies:
- Defects liability guarantee (1 year after handover): covers all defects reported by the homeowner during the first year, including odours caused by a minor execution fault (a badly installed siphon, a faulty seal, etc). The developer must carry out repairs at their own cost.
- Two-year guarantee (2 years after handover): covers fittings that can be detached from the main structure (taps, sanitary appliances) when they are rendered unfit for their purpose. If the odour comes from a faulty fitting, the two-year guarantee applies.
- Ten-year structural guarantee (10 years after handover): covers defects that compromise the soundness of the building or make it unfit for its purpose. A faulty buried foul-water network, a missing stack vent, or a structurally defective connection fall under the ten-year guarantee. The homeowner’s building damage insurance (dommages-ouvrage) then steps in to pre-finance the repairs.
An independent expert assessment precisely establishes the cause and which guarantee applies. Its cost is generally borne by the homeowner, but can be recovered from the developer or their insurer once liability is established. In a dispute, the judicial court can order a court-appointed expert assessment, but a prior amicable expert report often speeds up resolution of the case.
Frequently Asked Questions
The developer refuses to bring in an independent expert \u2014 what can I do?
You can appoint an expert of your choosing at any time. The expert produces a report you send to the developer by registered letter with acknowledgement of receipt. If the developer continues to deny the defect or refuses to make repairs, you can take the matter to the competent judicial court (Grasse for the Alpes-Maritimes, Toulon for the Var), either in summary proceedings or on the merits, backed by the expert report. The judge can order a court-appointed assessment or rule directly against the developer based on the amicable expert report.
The odours appeared after the defects liability period ended \u2014 am I still covered?
Yes, if the defect falls under the two-year guarantee (detachable fittings) or the ten-year structural guarantee (unfitness for purpose). Persistent sewage odours generally render the home unfit for its purpose, which triggers the ten-year structural guarantee. The expert assessment determines the legal classification of the defect and which guarantee applies.
How long does a sewage-odour expert assessment take in a VEFA home?
The site visit generally takes half a day to a full day depending on the size of the house, the number of sanitary fixtures, and the complexity of the network. Dye tests need an observation period (30 minutes to 1 hour per fixture), smoke injection requires preparing the network, and crawl-space inspection can take longer where access is difficult. The expert report is delivered within 10 days to 3 weeks.
Can the expert carry out the repair work after the diagnosis?
No. Cassini Expertise carries out no repair work and sells no materials. This complete independence guarantees the objectivity of the diagnosis and the admissibility of the report in any legal dispute. Remedial work is carried out either by the contractor appointed by the developer (if the guarantee applies) or by a tradesperson of your choice (if you have the work done yourself and then claim reimbursement).
Cassini Expertise: independent building expert in Antibes (06) and the Var (83)
With 15 years of experience in public works and site management followed by building expertise, Cassini Expertise covers the whole of the Alpes-Maritimes (06) and the Var (83) to assess defects in new-build VEFA properties, renovations, and existing buildings. As an authorised specialist expert, we use exclusively professional Trotec instruments and standardised protocols (dye tests, cold smoke, hygrometry).
We carry out no repair work, sell no materials, and have no ties to developers or tradespeople: our complete independence guarantees the objectivity of our reports, which are enforceable against insurers and admissible before the Grasse and Toulon judicial courts.
For any expert assessment request concerning persistent sewage odours, mould, or damp in your new-build VEFA home, contact Cassini Expertise on 04 22 46 06 04 or by email at [email protected]. We respond quickly to establish the origin of the defect and determine which guarantees apply.
Can legal aid fund an amicable expert assessment?
No \u2014 legal aid only covers the cost of a court-appointed expert assessment (ordered by a judge). For an amicable, contradictory expert assessment, funding is private. That said, if the amicable expert report is submitted during a later court-appointed assessment, its findings carry weight in the proceedings.
How does a contradictory expert assessment with the developer work?
The developer is summoned by registered letter with acknowledgement of receipt. They may attend or fail to respond. Either way, the site visit goes ahead and a formal record of findings is drawn up.
Who pays for the expert assessment if we win?
If the report demonstrates a defect falling under the defects liability guarantee, the cost of the assessment can be claimed back from the developer or their insurer.
Are wall probes destructive?
No \u2014 the non-destructive moisture meter measures moisture in materials by simple contact. Destructive probing is only considered as a last resort.
Related articles
- Building expert before purchase in the Var
- VEFA expert assessment in the 06 and 83
- G2 soil survey before purchase
What if the smell is « musty » rather than sewage-like?
A persistent musty smell in a new-build home points to a different family of causes: hidden damp feeding mould, sometimes invisible to the eye. In VEFA homes, three culprits dominate: a crawl space with standing water (spores rise up through the slab), a screed or plaster still damp at move-in (new-build construction takes months, not weeks, to dry out), and insufficient ventilation that lets moisture build up. The diagnosis sorts out which is which: a non-destructive moisture-meter survey of the walls, a thermal camera for cold spots, air hygrometry, and a crawl-space inspection. Measured evidence before any treatment \u2014 that’s the rule.
Cassini Expertise
Expert du bâtiment indépendant — Antibes (06·83·13·40)
📅 Mis à jour le 25/09/2026 — Cassini Expertise, expert bâtiment indépendant (06·83·13·40).