Unauthorized extensions on an otherwise legal property
In 30 seconds
A closed-in terrace, a pool, or an extra room built without a licence on a home that is otherwise legal is usually a problem you can fix, not a disaster. But you often find out about it at a bad time: when the square metres in the land registry, the Catastro, and the real building do not match. Correcting the record can also bring four years of back IBI.
What counts as an unpermitted extension?
Anything physically added to the property after its original licence, without a matching licence of its own. This pattern repeats along the coast in a small number of ways.A solárium is a private roof terrace on top of an apartment, usually reached by its own stairs and usually counted in the advertised area. It is one of the more common places this exact problem hides: whether anything built on it, a pergola, a room, an enclosed section, was ever permitted is a separate question from whether the solárium itself is legally part of your property, and an unpermitted structure on a roof is a defect a buyer inherits along with the terrace.
- An enclosed terraceGlass or walls added to a terrace or balcony that was originally open, to make extra indoor space.
- A swimming poolA pool built without its own separate licence. A pool needs this licence in addition to the house’s building licence.
- An extra room or an additional floorExtra space added beyond what the original licence allowed, sometimes built on a flat roof or a solárium.
- A boundary wall or outbuildingA structure on the plot that was never permitted, even when the house itself is fully legal.
How does a buyer discover this before completion, not after?
The sign to look for is a mismatch between two documents. These documents are supposed to describe the same property, but often do not. The Registro de la Propiedad records the legal description your lawyer reads on the nota simple. The Catastro is the separate tax-cadastre system that records the physical description used to calculate IBI. An extension built without a licence is rarely declared to either system, so the mismatch is often the only visible sign of it.Arevont’s technical-inspection partner walks through the property with the real floor plan. They measure what is physically there against what both documents say. This matters because a mismatch this small is easy to miss from a listing, a photograph, or even a normal viewing.
What the paperwork saysWhat is really built
- Registro de la Propiedad
- The legal description your lawyer reads on the nota simple. Rarely updated after an undeclared addition.
- Catastro
- The tax-cadastre’s physical description, used to calculate IBI. Sometimes larger than the registry, sometimes still showing only the original building.
- The property itself
- What is physically standing on the plot today. Only a walk-through with the floor plan confirms this.
| The mismatch pattern | What it usually means | How serious it tends to be |
|---|---|---|
| Catastro shows more square metres than the Registro | This is the most common pattern. A real extension was built and never declared to either system, so the tax record grew while the legal description stayed the same. | Worth investigating directly. This is the pattern that matters most here. |
| The Registro shows more than the Catastro | This usually means the Catastro record was simply never updated after a demolition or a boundary change. It is usually not a hidden addition. | Lower risk, but still worth confirming rather than assuming. |
| Both documents agree, but the property itself is larger | This is rarer and more serious. An addition was never declared anywhere at all, so a desk-based document check cannot find it. | Only a physical, on-site inspection against the floor plan finds this one. |
We describe the general Registro-Catastro coordination procedure as it normally works. This is based on several practitioner sources that describe the same process in the same way, not on one specific article of the Ley Hipotecaria. Your lawyer can tell you the exact provision that applies to your own property.
Can an old, undeclared extension be legalised now?
Often yes. The route is administrative, not a new building licence. You make a "declaración de obra nueva por antigüedad" through a notary, supported by a technical certificate of age. In Andalusia, you also file it through the Catastro’s own discrepancy procedure (Modelo 900D). This works because the same six-year time limit that applies to a whole illegal building under Andalusian law also applies to a single unpermitted addition. Once that time has passed on ordinary land, the addition can usually be registered as an existing fact instead of being demolished.The same exceptions apply here as to a whole illegal building, so check them rather than assume they do not apply. An extension on land under special protection, in the coastal influence zone, or affecting a registered easement does not get the six-year time limit at all, no matter how old it is. We explain the full set of rules, and the small group of land types where you cannot regularise, once, on the site’s separate whole-structure guide, instead of repeating it here.
- Get a technical certificate of ageA qualified professional confirms when the addition was built.
- Where
- An architect or a topographer
- Sign a declaración de obra nueva por antigüedadThe notarial deed that declares the addition as an existing fact.
- Where
- A notary
- File the Catastro discrepancy procedureModelo 900D updates the tax-cadastre record to match reality.
- Where
- Catastro
- How long
- Weeks, once the certificate and deed exist
- Confirm the land classification underneath the additionThe same protected-land exceptions that apply to a whole illegal building apply to a single extension.
- Watch out
- On protected or coastal land, this route is not available at all.
What people assumeWhat really has to happen
- If it is old enough, it automatically becomes legal
- It becomes registrable through a specific notarial and cadastral procedure, which someone has to carry out. Nothing happens automatically just because time has passed.
- A closed terrace or a small pool is too minor to matter
- Size is not what decides this. The same land-classification exceptions that apply to a whole illegal building apply to a single pool or terrace on the same plot.
If the current owner already tried to regularise the extension years ago and never finished, does the buyer take over that unfinished process?Generally yes, in practical terms. The underlying mismatch is still there until the process is fully completed, no matter who started it. Ask specifically what stage any earlier regularisation attempt reached: picking up a half-finished process is usually faster than starting one from nothing. But it is not automatically resolved just because someone once began it.
What does an undeclared extension cost if it is never fixed?
There are three separate costs, not one. None of them appears at the moment the extension was built.Arevont’s legal coordination requests the Catastro extract together with the nota simple, as a standard part of due diligence. This catches the mismatch while there is still time to negotiate it into the price, instead of finding it after completion.
| Where it costs you | How | How far it reaches back |
|---|---|---|
| Mortgage financing | A lender’s valuation uses the property’s registered square metres, not the marketed ones. So for financing, an undeclared extension does not count at all. | This does not go backward in time. It only reduces what a bank will lend today. |
| Back-owed IBI | Once a mismatch is corrected in the Catastro, whether by a future owner, a sale, or a town-hall check, the tax office can assess the unpaid tax debt for the past. | Up to four years back. This is the general time limit for a tax assessment. After the correction, the clock stops running further back. |
| Resale properties value | This is the hardest of the three costs to measure. An undeclared extension makes a future resale property slower, and can reduce what a careful buyer is willing to pay once they run the same check described above. | Ongoing, for as long as the mismatch is never corrected. |
Does an undeclared extension reduce the amount a bank will lend?It can. The mortgage valuation is normally based on the property’s legally registered size, not on what is physically there or what the listing advertises. If a property is truly larger than its paperwork says, the bank does not value the extra space until that space is properly declared.
Who should pay to fix this, the buyer or the seller?
In an ordinary negotiation, the cost of fixing a known undeclared extension is the seller’s problem, found before completion, not the buyer’s problem, found after it. Once a mismatch is found, it is normal and reasonable to ask the seller to either complete the fix before signing, or reduce the price by the estimated cost of doing it. This is better than accepting the property as-is and taking on both the cost and the paperwork.Neither Arevont nor a lawyer carries out the fix itself. A technical professional, usually an architect or a topographer, prepares the certificate of age and the plans. The notary and the Catastro’s own procedure handle the rest. What a lawyer’s coordination adds is good timing against the purchase, so the negotiation happens before you commit, not after.
- Option 1RecommendedSeller legalises before completionThe seller, who caused the problem or took it on from an earlier owner, fixes it before you own the property.
- Cost
- Seller pays
- Risk
- Lowest
- Speed
- Adds weeksFor the certificate, deed and Catastro filing
- Option 2Price reductionYou accept the property as-is, at a price that reflects the fix.
- Cost
- Priced in
- Risk
- MediumYou inherit the paperwork
- Speed
- No delay
- Option 3Buyer takes it on, unpricedAccepting the property without negotiating anything.
- Cost
- Buyer pays
- Risk
- Highest
- Speed
- No delay
What to confirm before you rely on these numbers
Four things, checked against your specific property rather than against a published range.| Confirm | What it tells you | What a bad answer looks like |
|---|---|---|
| Registered square metres against the Catastro figure | Whether the two systems, which are supposed to match, really match for this specific property. | Nobody has compared them, or an assumption that the marketed size is the legal one. |
| A physical walk-through against the real floor plan | The one check that catches an addition invisible in both documents at once. | Relying on the listing’s stated size without anyone standing in the property with a plan. |
| Whether any addition has AFO status or a completed declaración de obra nueva por antigüedad | Whether an existing mismatch has already been resolved through the proper procedure, or is still open. | A verbal claim that "it is fine" with no document behind it. |
| The land classification under any addition, checked on its own | The same protected-land exceptions that apply to a whole illegal building apply to a single extension too, not only to a whole building. | An assumption that a small addition is automatically too minor for the exceptions to matter. |
This is usually fixable, and worth checking anyway
This is general information about how an unpermitted extension is treated under Andalusian and Spanish tax law, current as of 17 August 2026. We read the registry-versus-cadastre mismatch process across several practitioner sources that describe standard practice. The six-year time limit is shared with the site’s whole-structure guide, and carries the same lawyer-confirmation flag as that page. This is not legal advice about any specific property.Most findings of this kind are exactly what we describe above: you can fix them, you can price them, and they are normal enough on this coast that they are rarely a reason to decide not to buy an otherwise good property. The real mistake is not the extension itself. It is finding out about it after completion instead of before, when the only option left is paying for it entirely yourself.This page does not perform the check it describes. Arevont’s technical-inspection partner and its legal coordination service check the specific property for you, free of charge, before you pay a reservation.
Most common questions about unauthorized extensions
- Is a closed-in terrace always an unpermitted extension?
- Not always. Some developments include an enclosed terrace in the original licensed design from the start. In that case, it is simply part of the legal property. The real question is whether the specific enclosure on the specific unit was part of what was originally permitted. This is exactly what the registry-versus-cadastre comparison and the physical inspection check.
- Does a swimming pool always need its own separate licence?
- In most municipalities, yes. A pool needs its own building licence, separate from the house. An older pool built without one is one of the more common findings on this coast. It follows the same six-year and land-classification rules as any other unpermitted addition.
- If I buy a property with a known, priced-in unpermitted extension, does that debt or risk pass to me?
- The physical situation passes to whoever owns the property. This is exactly why it should be priced in, not ignored. Any back taxes assessed before your purchase are usually the previous owner’s responsibility to pay at completion. But the ongoing legal status of the extension becomes yours to manage from then on. This is why you should negotiate the price, not just accept a lower one without understanding why.
- How is this different from the general Registro-versus-Catastro mismatch described elsewhere on this site?
- That page explains the general split between the two systems, and why they can disagree for many reasons, including simple administrative delay. This page covers the specific, most serious version of that mismatch: one caused by a physical addition that was never declared to either system, with its own cost and its own fix.
