Short answer: measurements and legal status first, use second
The space under a pitched roof can become a bedroom, a study or a guest room, but only the part with enough headroom, a floor that can carry a residential load, and heat and cold that insulation can realistically control. The low zones under the slope become built-in storage. You decide the use after measuring and checking the paperwork, not before.
The classic mistake: the owner of a house in Castelldefels sees one more room in the loft, orders furniture, and then finds you can only stand upright in a strip about 1.8 metres wide, and in summer it gets close to forty degrees up there.
A headroom map instead of an impression
Draw a plan of the loft and split it into three zones: where you can stand, where you can sit or lie down, and where only a box will fit. Measure with a tape or laser every metre, allowing for finished floor and ceiling rather than bare rafters. Insulation and a new floor will take a few centimetres off the top and bottom.
The headroom above which floor area counts as usable for a habitable room is set by regulations, and how they apply to your house is for the architect to confirm. For the owner the practical question is different: where the bed goes so you do not hit your head getting up, and where the desk goes so you can sit up straight at it.
The floor and the roof structure
A loft floor was often designed for roof maintenance and light storage, not for a bedroom with wardrobes, books and people. Before laying parquet you need an inspection and, where necessary, calculations from an engineer. In an older house with timber joists this step is not optional.
The second trap is gaining headroom by cutting out ties, struts or part of the roof framing. Those members hold the roof up. Any change to structural elements happens only with a design from a qualified professional, and it will almost certainly need a permit from the ayuntamiento.
In Barcelona the heat matters more than the cold
On the coast the main problem with a loft room is summer, not winter. The roof heats up all day and releases it at night, so a bedroom under the roof without good insulation stays stuffy until morning. The fix has three parts: generous insulation along the slope, a ventilated gap under the roof covering where the structure allows, and external shading on the windows.
Air conditioning is needed as well, but it treats the symptom. Decide early where the indoor unit will sit, how the refrigerant pipes will run and where the condensate will drain: space under a slope is tight, and those details determine where the wardrobe can go later.
Light, air and a bathroom under the slope
Roof windows let in noticeably more light than a vertical window of the same size, but they also overheat more. Choose them together with external shading and think about how you will open them if they sit high. For airflow, two windows on different slopes or facing different ways work better than one large one.
A bathroom under the roof works if fittings are placed by headroom: shower and basin where you can stand, the WC can go in a slightly lower zone. An extractor is essential, and its outlet through the roof has to be detailed with the roofer so it does not become a leak.
Low zones and access
Everything below the height of a seated person should go to made-to-measure storage: drawers under the slope, cupboards with doors that follow the pitch, closed recesses for equipment. This joinery is measured after finishing and uses dead corners that would otherwise just collect dust.
A separate question is how you get up there. A loft ladder is fine for a storeroom, not for a bedroom used every day. A fixed staircase needs room below, and that has to be considered when planning the floor underneath.
Paperwork and neighbours
Turning a non-habitable loft into a room can change the usable floor area of the house and require a licencia de obras. Which type of permit you need and how it affects the property's documents is confirmed with the ayuntamiento or your architect. In an apartment building the loft is often a common element, and it cannot be touched without a decision by the comunidad de propietarios.
We start these projects with a site visit and measurements, and the price comes in the estimate that follows. As a guide by finish level: from 1,000 to 3,000 EUR/m² and up, excluding VAT, depending on whether it is Basic, Standard or Luxury.