In short: what decides it
A single-storey house is easier to live in without stairs and gives direct garden access from any room, but it takes up more land and needs more foundation and roof per square metre of living space. A two-storey house leaves more garden and often opens up views from the upper floor, but adds a staircase that eats area on both levels.
Most often the plot itself decides: how much of it can be built on, how tall the house can be and how steep the land is. So the comparison starts not with pictures but with the planning parameters of that specific plot.
Plot parameters first
The municipal planning scheme sets the plot's maximum site coverage, buildable area, height and number of storeys, and setbacks from the boundaries. On a narrow plot in Castelldefels, the setbacks can leave a footprint where a single-storey house with the bedrooms you need simply does not fit. On a slope in the Garraf the house naturally falls into two levels, and the debate becomes how to fit them to the terrain.
These parameters are checked before the first sketch, using the ayuntamiento's data for that plot. What else to check when buying a plot is covered separately. If you are still choosing land, decide first which house you want and look for plots that suit it.
Compare the same brief
Two options can only be compared with the same brief: the same number of bedrooms and bathrooms, a study, storage, a plant room and a garage. If one has a study and the other does not, the difference in area and cost tells you nothing.
Ask for plans showing furniture, circulation and storage, not just room outlines. On a single-storey plan, check corridor lengths: with a large footprint they grow, and area disappears into circulation. On a two-storey plan, see where the stairs sit and how much room is left at the bottom and the top.
Stairs: area, safety and the future
A staircase with its landings takes several square metres on each floor, and you pay for those metres as living space. A comfortable stair with shallow risers takes more room than a steep one, and this is not the place to save: it is used every day, with children, shopping and laundry baskets.
Think about who will live in the house in ten years. If elderly parents might move in, or you are building to stay for a long time, a two-storey house benefits from a ground-floor bedroom with a bathroom. Until it is needed, it works as a guest room.
Where the cost difference lies
A single-storey house has more foundation and more roof per square metre of living space, and a roof with waterproofing and insulation is one of the expensive parts of the envelope. A two-storey house adds an intermediate floor slab, the staircase, scaffolding for work at height and more facade relative to its footprint.
So a generic price per square metre is no use for comparison. You need a breakdown by main chapters for each option: foundations, structure, roof, facade and windows, services and finishes. Only then can you see where one option costs more than the other and why. How a new-house budget is structured is covered in a separate article.
Light, views, garden and solar panels
In Castelldefels and the Garraf, an upper floor often gives views of the sea or hills over the neighbours' fences, and upstairs bedrooms gain privacy. A single-storey house, by contrast, is more closely tied to the garden and pool: the terrace extends the living room and in summer the house lives outdoors.
Roof area affects solar panels. On our house with a pool, built start to finish between 2022 and 2024, the panels were part of the design from the beginning. A single-storey house has more roof and usually ample room for panels. A two-storey one has less, so it is worth checking panel positions on the sketch together with the orientation of the house.