When buying a property in Spain, especially in the sun-soaked Costa Blanca, you may come across villas with beautifully transformed underbuilds — lower levels that have been turned into stylish guest apartments, independent suites, or even rental spaces.
This kind of smart renovation makes perfect sense in a region where indoor-outdoor living and flexible space are part of the Mediterranean lifestyle. But if the conversion isn’t yet reflected in the property’s paperwork, many buyers wonder — can it be legalised?
The good news is: in most cases, yes, it can. And once legalised, this extra space can significantly increase your property’s comfort, value, and potential.
An underbuild (or sótano) is the lower level of a home, typically designed as storage or a garage. In Costa Blanca villas, these spaces are often bright, dry, and perfectly suited for creative transformations — guest apartments, entertainment areas, or independent living quarters for family or visitors.
Many homeowners have already taken advantage of this, turning their underbuilds into charming, fully equipped spaces that add real character and functionality to the property.
Sometimes, these transformations are made without updating the official records (escritura and catastro). This doesn’t necessarily mean the space is “illegal” — it simply means it hasn’t yet been registered as habitable space.
Common reasons include:
The work was completed many years ago and documentation wasn’t updated.
The previous owners didn’t apply for a formal license.
The area wasn’t originally included in the total built size on the title deed.
Fortunately, in most cases, these situations can be resolved smoothly with professional guidance.
Legalising a converted underbuild is a well-established process in Spain, and when handled properly, it adds long-term value and peace of mind.
Here’s how it usually works:
1. Confirm Local Planning Rules
Your first step is to check the local urban planning regulations. Every town hall (ayuntamiento) — for example, in Jávea, Moraira, or Altea — has its own criteria, but most are open to regularising existing conversions, especially when the structure is safe and well built.
2. Hire an Architect or Technical Engineer
An arquitecto técnico will visit the property, confirm the structure complies with building standards, and prepare the technical report (certificado de legalización or certificado de antigüedad).
If the works were completed more than four to six years ago (depending on the municipality), the property often qualifies for retroactive legalisation since the timeframe for any sanctions has expired.
3. Register the Extra Space
Next, your lawyer can help update the title deed through a Declaración de Obra Nueva (Declaration of New Works). This officially adds the converted space to the escritura and catastro, ensuring your property documents reflect the true layout.
4. Obtain the Habitation Certificate
The final step is obtaining the cédula de habitabilidad or licencia de ocupación, confirming that the space meets the standards for living — such as ventilation, light, and safety. This also allows the space to be used legally as guest accommodation or for long-term rental.
In rare cases, if the property exceeds the allowed building size (edificabilidad) or is on protected rural land (suelo rústico protegido), full legalisation may not be possible.
Even then, the underbuild can often be used privately, providing valuable extra space for family, guests, or personal leisure — simply without official registration as a separate dwelling.
Taking the time to legalise a converted underbuild offers many advantages:
It increases the official value of your property.
It ensures security and peace of mind when selling or renting.
It makes utility connections and licensing straightforward.
And most importantly — it helps you enjoy your home with full confidence that everything is in order.
If you’re buying a property with a converted underbuild:
Request the current escritura and catastro plans to see what’s officially registered.
Ask for any building permits or certificates related to the renovation.
Consult a local architect and a qualified lawyer to confirm that legalisation is possible — in most cases, it’s a simple process with clear steps and predictable costs.
An underbuild conversion is one of the most attractive features of many Costa Blanca homes — offering versatility, privacy, and extra comfort. And even if it’s not yet legalised, there’s usually a clear and achievable route to make it fully compliant.
With expert local guidance, the process is straightforward, and the result is a property that’s more valuable, more secure, and ready for any lifestyle you envision.
At La Boutique Luxury Real Estate SL, we collaborate closely with architects, lawyers, and local authorities to ensure each home we represent meets the highest standards of legality and quality. Because true luxury lies in the details — and those details start with trust, transparency, and care.
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