1 in 4 homes has a serious defect that isn’t visible to the naked eye, with an average repair cost of over €8,000. And only 15 % of people ask for a technical review before buying.
This list of the most common defects in a resale home doesn’t tell you what the one you’re looking at is sure to have. It may well be perfect, and often it is: what an inspection adds is the certainty of checking it before signing, not a guaranteed defect. With that said, this is what shows up most, how it’s documented, and what’s usually simple to resolve compared with what deserves discussing before agreeing the price.
What are the most common defects in a resale home?
- Damp.
- Fissures and cracks.
- Old electrical installation.
- Old plumbing and drainage.
- Exterior window frames without a thermal break.
- Insufficient ventilation.
- Ageing roofs and flat roof terraces.
Damp
Most frequent: stains on ceilings and walls, especially under terraces, in bathrooms and on façades that get little sun. They’re documented with a moisture meter and a thermal camera. The same damp pattern can be consistent with water getting in through the roof or façade, with condensation due to poor ventilation, or with a plumbing fault, and deciding which one it is without a specific diagnosis isn’t reliable for anyone, qualified or not.
What’s usually simpler: if the diagnosis confirms condensation due to lack of ventilation, improving extraction is sometimes enough. What weighs more in negotiation: water ingress from the roof or façade, because it means working on the building envelope or on common elements, not just inside the home.
Fissures and cracks
They appear mainly in walls and where different materials meet (a partition against a column, render against a window frame). They’re measured with a crack gauge, photographed, and their pattern is described: direction, width, length. Assessing a building’s load-bearing capacity falls under the Structural Safety basic document of Spain’s Building Technical Code and a qualified professional, and it requires testing or an exploratory opening, not a glance. A crack can be consistent with temperature movement, with badly applied render or with a structural issue; only an instrumental diagnosis tells them apart.
The buyer’s usual mistake: being equally alarmed by a fine, superficial fissure and by a diagonal crack several millimetres wide. They don’t all carry the same weight, and that’s precisely why an inspection doesn’t settle the cause without testing, but it does document the pattern precisely enough to decide whether it’s worth requesting that diagnosis before buying.
Old electrical installation
Very common in homes that haven’t had a full renovation in decades: consumer units with few circuits, sometimes without a residual current device. Spain’s Low Voltage Electrotechnical Regulation (ITC-BT-25) requires homes to have a main switch, one or more 30 mA residual current devices and independent circuits by use (lighting, general sockets, cooker and oven, washing machine, bathrooms). Verifying whether a specific installation complies is work for an authorised electrician; counting the circuits and checking whether there is an RCD is an observable fact that any inspection should record in writing.
Why it matters more than it seems: renewing a complete electrical installation is a significant item in any renovation, and one of the least visible during a sales viewing: the consumer unit may be behind a door that nobody opens.
Old plumbing and drainage
Pipes made of materials no longer used, leaks at old joints, and drains without a trap or badly connected. On lead there’s a clear reference: Royal Decree 3/2023 prohibits installing it in contact with drinking water and requires those elements to be replaced when repair work is carried out on the installation. Drainage problems usually show up as smells or repeated blockages, and are checked with a simple functional test: run the water and see that it drains.
Exterior window frames without a thermal break
Old aluminium windows without a thermal break let cold and heat through the frame itself, and the thermal camera shows this as a band of different temperature around the window. It’s one of the most direct observations in the whole inspection: you see it with the instrument.
Insufficient ventilation
Interior bathrooms and kitchens with no window and insufficient mechanical extraction. Spain’s Building Technical Code (DB-HS 3, indoor air quality) now requires new homes to have a general ventilation system, with extraction in bathrooms and kitchens; many older homes don’t have one. A lack of air renewal encourages condensation and, with it, some of the damp from the first point on this list.
Roofs and flat roof terraces
In detached houses or penthouses with their own roof terrace, waterproofing ages, and the junctions with parapets, drains and joints are where problems most often appear. It’s the kind of detail you only see by going up to look, not from the listing.
The real advice
The defect that looks most alarming at first sight, a large damp stain or a visible crack, isn’t always the most expensive to fix. Quite the opposite: what really drives up the cost of a renovation is usually where nobody looks during a sales viewing: inside the consumer unit, behind a wardrobe, under the flooring. An inspection isn’t worth it for confirming what you can already see; it’s worth it for documenting what isn’t visible at a glance and is going to cost money.
The price of an inspection doesn’t depend on what’s found: the service is the same whether the home is perfect or something turns up to negotiate.
Sources
- Real Decreto 842/2002, Reglamento Electrotécnico para Baja Tensión (ITC-BT-25) — BOE
- Real Decreto 3/2023, criterios técnico-sanitarios de la calidad del agua de consumo (art. 45 y disposición adicional duodécima) — BOE
- Documento Básico HS Salubridad (HS 1 y HS 3) — Código Técnico de la Edificación
- Documento Básico SE Seguridad Estructural — Código Técnico de la Edificación
If you’re about to buy and want to know what condition the home is in, let’s talk it through — no obligation.