Buying a house in Spain with an undeclared pool or extension: what you're taking on and how it gets fixed

The scene I see almost every week

A couple arrives at my office, usually from Northern Europe, excited about a house on the coast. A villa with a garden, a lovely pool, an enclosed porch that the seller uses as a winter lounge. All of it beautiful. And then I ask for the paperwork, and the usual problem shows up: the pool appears nowhere, the winter lounge is still, «on paper», an open terrace, and the house has thirty square metres more than the Registro says.

This is extremely common in Spain, especially in villas, rural properties and older homes on the coast. And here is the important part: having undeclared building work does NOT mean you cannot buy. It means you need to know exactly what you are buying, because this is not a minor administrative detail. Let me walk you through it calmly.

Not all undeclared building work is the same: the four situations

When someone tells me «the house has undeclared building work», the first thing I do is work out which of these four situations we are in. Because the difference between one and another is the difference between a calm formality and a serious problem.

1. Legal construction, just not registered

The best of them all. The construction was carried out at the time with its municipal licence, but somebody forgot to take it to the Registro de la Propiedad. The construction is legal; the registration paperwork is simply missing. Here the fix is usually straightforward: the documentation is submitted and it gets registered. Zero drama.

2. Old, time-barred construction that can be registered by prescription

The construction or extension had no licence, but it is old enough that the authorities can no longer order it demolished, because the time they had to act has run out. In that case it can be registered at the Registro «by prescription», with a technical or Catastro certificate proving that the building was finished by a certain date. Careful: being able to register it does not mean it is fully legal. I come back to this in a moment, because it is the point that causes the most confusion.

3. Non-conforming construction (fuera de ordenación)

The construction exists and can remain standing, but it clashes with current planning rules. It can usually be sold, but it carries significant limitations: often only conservation works are allowed, you cannot extend it, and if it is ever demolished or falls into ruin you may not be able to rebuild it in the same way. It is buyable, but you have to go in knowing exactly what you are dealing with.

4. Construction that is not yet time-barred, or is never time-barred

The worst case. Either the construction is recent and the authorities still have time to act, or it sits on land where the passage of time NEVER protects it (protected non-developable land, public domain, green zones…). Here the risk is real: a disciplinary file, a fine, an order to legalise, or even a demolition order. This is the situation where I tell you, in no uncertain terms, not to sign anything without having it reviewed first.

Registering by prescription is NOT legalising (this is the key point)

This is the most expensive misunderstanding I see. Many people believe that if a construction can be registered at the Registro «by prescription», everything is already sorted and legalised. That is not the case. They are two different things running on separate tracks.

The route of new-build registration by prescription (set out in art. 28.4 of the national Land Act) allows an old construction to be recorded at the Registro once the authorities can no longer order its demolition due to the passage of time. That gives you something very specific: the ability to sell, mortgage or inherit the house normally. But it does NOT turn the construction into something legal from a planning standpoint, nor does it remove its limitations. The planning situation remains exactly what it is.

The most important thing for you as a buyer: registering the construction does not «hide» anything. Quite the opposite. When an old construction is registered without a certificate from the Ayuntamiento, the registrar is required to notify the Ayuntamiento itself, which can then record the construction's specific planning status and its limits at the Registro. In other words, registering triggers a formal notice to the town hall. It is not a rug to sweep the problem under.

The Valencia region's caution: non-conforming status and rural land

Since I work in Calp, I am speaking about the rules that apply here, in the Comunitat Valenciana, where the reference law is the Ley de Ordenación del Territorio, Urbanismo y Paisaje (TRLOTUP). But keep one thing in mind from the start: planning law is regional and municipal, so what I am telling you is the Valencian rule, and the specific case always has to be checked with each Ayuntamiento.

The general rule in the Comunitat Valenciana is that the authorities have a deadline — around fifteen years from the date the construction was fully completed — to require it to be legalised or restored to lawfulness if it was built without a licence. But do NOT stop at that figure alone, because it has very important exceptions, and this is what catches people out the most.

That deadline does NOT apply — meaning the authorities can act at any time, with no time limit — when the construction sits on non-developable land, land or buildings in the public domain, green zones, roads, open spaces, public amenity uses, or assets forming part of the Valencian cultural heritage. This is decisive for villas, chalets and properties on rural or protected land: age does NOT always save you there. A twenty-five-year-old pool on protected non-developable land can still be liable to demolition.

And the final caution: even once the deadline has passed, the Valencian rule itself makes clear that the mere passage of time does not legalise the construction. It is still illegal, just no longer demolishable; and while that situation lasts you will not be able to carry out reforms, extensions or consolidation works on what was built without a licence. That is why «non-conforming» and «rural land» are, to me, two red flags that call for looking closely at the file.

Registro, Catastro and reality: the Catastro does not legalise anything

To understand what you are buying, think of the house as having three «photographs» that should match: physical reality (what is actually built), the Registro de la Propiedad (who owns it and what charges it carries) and the Catastro (what the tax authority has on record for taxes such as IBI). When the three line up, everything runs smoothly. When they do not, doubts start creeping in — from the bank, from the insurer, and from the buyer themselves.

And here is the most common mistake I hear: «the pool is in the Catastro, so it must be legal». NO. The Catastro describes the property for tax purposes, but the Catastro does NOT legalise anything from a planning point of view. Paying more IBI because of extra square metres does not mean those metres have a licence. The Catastro can help prove that the construction exists and since when, but it does not replace a municipal licence.

What I see every week is exactly this mismatch: the pool shows up in the aerial photo but not in the Catastro; the Catastro records more floor area than the Registro; the Registro speaks of 120 square metres while the real house has 180; the title deed does not even mention the pool; and the bank's valuation exposes it all at the worst possible moment. That is why, before buying, all three photographs — plus the situation at the town hall — need to be laid side by side.

What risks you take on if you buy without checking

Let me lay them out clearly, because it helps to have them all in front of you before deciding:

• Planning risks: an order to legalise, a disciplinary file, a fine and, if the authorities' power to act has not lapsed or is never subject to a deadline, even a demolition order.
• Use risks: trouble obtaining an occupancy licence (first or second occupation), letting the property, offering it as a holiday rental, renovating it, or even setting up utility supplies.
• Registration risks: the Registro refusing to register the pool or the extension, requiring georeferencing, or recording a note flagging an unfavourable planning status.
• Financial risks: the bank may value only the declared part and cut or refuse the mortgage, and the insurer may exclude whatever is undeclared.
• Tax risks (as a guide): the Catastro may need updating and the IBI may change, and declaring the construction can trigger its own tax on the deed; it is worth quantifying this before buying, not after.
• Resale risks: down the line, your own buyer will demand exactly what I recommend you demand today — everything regularised, or a lower price.

How to protect yourself before signing, and how it gets fixed

This is the order I follow when a buyer brings me a house with building work that does not add up. The idea is simple: check before signing, and put in writing what happens if something cannot be fixed.

1. Request the nota simple (Land Registry extract) and the Catastro certificate, and compare them against each other and against the real house: do the floor area, description and annexes match?
2. Ask the Ayuntamiento about the planning situation: existing licences, whether there is an open disciplinary file, what the local plan says, and how the land is classified.
3. Check the land classification with particular care: if it is non-developable, protected, or public domain land, that is a red flag, because age may not protect you there.
4. Instruct an architect or building surveyor to measure the real construction, establish its age, and state whether it can be legalised, registered by prescription, or neither.
5. Based on that diagnosis, choose the route: ordinary planning legalisation if it fits the local plan; new-build registration by prescription if demolition is no longer possible; or renegotiate the purchase if the situation is delicate.
6. Protect the purchase contract: build conditions into the deposit agreement or the contract so the seller regularises the situation before signing, or hold back part of the price, with the option to withdraw if it cannot be fixed.

If you have fallen in love with a house in Spain and it has a pool, a porch, or a few square metres that do not quite add up on paper, write to me and we will look at it together before you sign anything. This is not about scaring you off or killing the deal: a great many of these houses are bought perfectly well — you just need to know which of the four situations you are in, and have it all tied down. I would rather help you buy with your eyes open than put out a fire afterwards.