The short answer: the plot's parameters decide
You can extend a home exactly as much as the local planning rules allow for that specific plot, minus what is already built on it. Step one is getting those figures from the ajuntament; drawing comes later.
An empty lawn behind the house tells you nothing about the margin. A house in Castelldefels on an 800 m² plot may already have used up all its permitted floor area, while the identical house next door can add a whole storey because the two plots fall into different planning zones.
Which parameters to look at
Edificabilidad (buildability): how many square metres of floor area can be built on the plot in total. Everything existing is subtracted from it, including terraces enclosed years ago and outbuildings, if they are on record.
Ocupación (site coverage): what percentage of the land the building may cover in plan. You can have spare floor area but no spare coverage, and then the only way is up.
Height and number of storeys. Adding a floor is only possible if the zone allows one more level.
Retranqueos (setbacks): the minimum distances to the boundaries and the street. Most extensions planned right up against the neighbour's fence are ruled out by this alone.
The figures come from the ajuntament's planning report or certificate and from an architect who knows how to read it. How to request it and how long it takes varies by municipality and is confirmed there.
Four ways to extend a house
Ground-floor extension. The most straightforward route if there is spare buildability, coverage and setback. It needs new foundations and a well-resolved junction between the new roof and facade and the existing house.
Adding a storey. It takes no land, but it loads the existing walls and foundations. Nobody should design one without first surveying the structure.
Basement or loft. Depending on the municipality's rules, these areas sometimes do not count towards buildability and sometimes count in full. In a basement the issues are damp and ventilation; in a loft, headroom and roof insulation.
Enclosing a terrace or porch. It looks minor, but it usually adds built floor area and goes through the same process as an extension.
The existing house's structure and services
Having square metres available on paper does not mean the house can carry them. For an extra storey, the architect or engineer assesses the foundations, load-bearing walls and floors; sometimes ground data are needed and a geotechnical study is commissioned.
The second thing people forget is building services. A boiler or heat pump sized for 180 m² will not heat 240. The same goes for the electrical supply, roof drainage and sewer connection. If the plot has a pool or solar panels, their position also affects where the new volume can go.
The permit and the paperwork afterwards
An extension that adds floor area almost always needs a major-works licence with an architect's project. Review times and fees depend on the municipality and are confirmed with the ajuntament, so they go into the programme with a buffer from day one. The procedure is covered in our article on the llicència d'obres majors.
Once the build is finished, the new area has to reach the paperwork: usually a declaration of the extension before a notary, registration at the Property Registry and an update at the Cadastre. The order and costs are confirmed with a gestor or the notary. Unregistered floor area always surfaces at the worst moment: when you sell the house or apply for a mortgage.
Mistakes you cannot fix later
Building first and legalising later. If the extension exceeds the plot's parameters, there may be no way to legalise it, and the conversation turns to demolition.
Commissioning the design before you have the planning report. A lovely living room opening onto the pool is useless if it sits inside the setback strip.
Assuming an enclosed terrace does not count as floor area. For the ajuntament it almost always does.
Forgetting the services and ending up with a new room you have no way to heat.
Where to start
Send us the plot address or the referencia catastral, the house drawings if you have them, and tell us how much space you need and what for. We check the parameters and structure first; architecture comes after.
We build homes in Barcelona and its province from start to finish. The private house with a pool and solar panels that we delivered between 2022 and 2024 went through every stage, from design to handover. Each project has one person in charge, we work in Russian or Spanish, and payments are staged according to the contract. We reply to enquiries within an hour.