What is the difference between net floor area and gross floor area when buying a house or valuing it?

Buying a house and analysing its square footage.

Thinking of asking for a mortgage to buy a house, or selling yours?

When going down either path, a key factor is the floor area of the property. It doesn't matter what price we have in mind or what we are willing to offer or ask for. It doesn't even matter if we research the prices of other properties in the area: the most accurate way to see whether a price is competitive is to calculate the price per square metre of the house.

But finding this information can sometimes be tricky. When buying, it's easy to find a lot of overpriced properties. The seller could add a few extra metres to the floor space, thus lowering the price per square metre and making it look more attractive. But we advise against this practice, because the buyer will eventually find out the real size of the property and we might lose the sale. It is best to find out the floor area listed at the land registry or in the deeds, although if the property is old there may be the odd mismatch and some elements may not have been calculated: terraces, attics, patios...

There is another big difference that we need to be aware of:

• The net, or usable floor area of a property, meaning the area we can actually walk on inside the home (including wardrobes). In this case, partition walls, falsework and pillars do not count. This information is important because it gives us a good idea of how much space we will have to live in, although it is difficult to know without seeing a floor plan.
• The gross floor area of a property. This means the total number of metres within the perimeter of the property, including partitions, electrical installations, drains, and so forth, but excluding spaces that are under 1.5 metres in height in residential areas. Meanwhile, if walls are shared with a neighbour, the perimeter will be calculated from the midpoint of these party walls. So, while gross floor area is still used as the reference for valuing the property, it is not the same as the actual amount of space one can use within the property.
• Gross floor area with proportional share of communal areas, such as a paddle tennis court, a swimming pool or even the corridors in the apartment building. This is the floor area on record at the cadastre and in the deeds, showing the floor area of the home and separately the shared ownership ratio of the communal areas.

What about patios and terraces?

These areas often cause confusion, given the growth of housing developments that include terraces or patios on ground floors with gardens, flats and penthouses. The cadastre calculates them at 50% of their floor area if they are covered, and at 100% if the element is enclosed on three sides or more. However, in the case of council or government subsidised housing, these elements will never count for more than 10% of their total floor area. This ratio is used for registry and tax purposes, although in practice it is not particularly useful in showing what the property is really like.

In short, if we are looking to buy or sell a house, it's best to show the floor area of the property and of the terrace separately and to be very clear and explicit so as to avoid nasty surprises down the line. Do not confuse net floor area with gross floor area, find out whether the terrace is enclosed, do not count the floor space of garages or storage rooms and, obviously, do not make approximate calculations or round up or down.

How to calculate the floor area of a property?

It is extremely useful to have an architect on hand to help us determine each type of floor area and to measure the house properly. If, however, we plan to do it ourselves, here are some tips: the first step is to measure and multiply the length and width of each wall in each room, and then deduct windows and doors. Hopefully we can find this information on the website of the cadastre.