In short: safety and space first, design second
An interior staircase in a duplex has to pass on safety and geometry first and on looks second. Riser height, tread depth, flight width, headroom and balustrades decide how much space the stair takes on both floors. Material and shape are chosen within those limits.
The mistake we see most often is drawing the staircase last, once both floor plans have been signed off. It ends up too steep, too narrow or cutting into a walkway. The right order is the reverse: position and type of stair first, everything else around it.
The geometry you cannot squeeze
An old rule describes how comfortable a stair is: twice the riser plus the tread roughly equals an adult's stride, about 62–64 cm. A step 17–18 cm high with a 28–29 cm tread is easy going. Push the riser to 21–22 cm to shorten the flight and walking down with a cup of coffee becomes unpleasant, and for children or older people, dangerous.
Minimum requirements for stairs and balustrades are set by the Código Técnico de la Edificación, in its safety-in-use document (DB SUA). The project technician checks the exact values for your case, and we advise against designing right at the limit: the regulation is a floor, not a target.
Check headroom separately. The opening in the floor slab has to be long enough for a tall person not to catch the edge on the way up. That opening is what takes area from the upper floor, and it gets forgotten when people only measure the stair's footprint below.
Stair types and the space each one takes
A straight flight is the most comfortable and the simplest to build, but it needs length. L- and U-shaped stairs, with a landing or winders, are more compact and give you a resting point halfway. Winders save metres, but they are harder to walk on at the narrow end, so their width is checked along the walking line.
A spiral stair takes the least floor area, but you cannot carry a sofa up it and laundry is awkward. It makes sense as a secondary stair, say to a roof terrace, not as the main link between two living floors.
Cantilevered treads fixed into the wall look light, but the wall has to take that load. In an older building that means a structural check of its own, and sometimes a hidden steel section.
Permits and structure: when you need a project
If you are joining two flats vertically or cutting an opening in a floor slab, you are altering the building's structure. As a rule that work needs a technical project and a permit from the Ajuntament, and in an apartment building the agreement of the comunidad de propietarios as well. The permit type and timescales are confirmed with the Ajuntament or a gestor.
In a detached house, say in Castelldefels, the stair is part of the project from the start: the opening, edge reinforcement and fixings are calculated along with the rest of the structure. Moving it once the slab has been poured is expensive and slow.
Materials, balustrades and lighting
Solid oak treads, microcement over a concrete base, a steel stringer with timber treads: all of these work in a high-end interior. When choosing, think about sound and wear. Steel without damping pads rings underfoot, and polished stone is slippery in socks.
A glass balustrade takes up no visual space but needs precise fixings and frequent cleaning. Vertical balusters are safer than horizontal rails, which a child can climb. The handrail should sit comfortably in the hand and run continuously along the whole flight.
Lighting the treads with wall lights or an LED strip under the nosing is first of all about safety at night. Put switches at the top and bottom. The space under the stair works well for built-in storage, but leave access to any services or wiring that run through it.
What to check before ordering the staircase
The order is: position and type of stair on both plans, size of the slab opening allowing for headroom, step calculation based on finished floor levels rather than bare concrete, fixing method, balustrades, lighting and, last of all, material. Measurements for manufacture are taken after the screed and allow for the thickness of the final floor; otherwise the first or last step ends up a different height. It is one of the most noticeable mistakes, because your foot feels it straight away.
If you are planning a duplex in Barcelona or a house on the coast, send us the plans of both floors. We reply within an hour, come to measure and show you where the stair takes the least usable area without giving up comfort.