What does "built-up area" mean — and why is it smaller than my house?
Built-up area is the house's footprint on the plot, seen from directly above. If the house has two storeys, it is roughly half the living area — and that is not a mistake.
Short answer: built-up area (the footprint) is the area the building covers on the plot — the shape seen from straight above, measured on the outside. Living area (floor area) is every storey's living space added together. A two-storey house with 160 m² of living area will therefore typically have a footprint around 80 m². The two figures are not meant to match, and a single-storey house is the one case where they come close.
The footprint, not the floor
Imagine hovering in a drone directly over the house and tracing the outline of the roof — or really of the walls beneath it — down onto the plot. That shape is the built-up area. It tells you how much of the plot the building occupies, not how much of it you can actually live in.
That is why an upper storey does not add anything: it sits inside the very same footprint. And that is why the built-up area of a two-storey house is roughly half its living area.
Built-up area is measured on the outside — to the outer face of the exterior walls. That applies to most official areas in a public building register, and it is the whole reason the register's figure and your own internal measurements can never come out the same.
The arithmetic
A house with two full storeys, each measuring 8.00 × 10.00 m on the outside:
- Built-up area: 8 × 10 = 80 m²
- Total floor area: 2 × 80 = 160 m²
- Your own measured floor (internal, 35 cm exterior walls, 22 running metres of internal walls per storey): roughly 2 × 65 = 130 m²
Three figures, three questions, the same house. None of them is wrong, and all three are useful — just not for the same thing.
Notice how easy it is to draw the wrong conclusion here: compare your measured 130 m² floor with an 80 m² built-up area, and it looks as if the register is missing 50 m². Read the field name, not just the number.
What counts towards the built-up area?
As a rule, it is the building's footprint that counts — whatever actually stands on and occupies the plot. An extension that goes beyond the house's existing outline expands the built-up area, while an attic storey or an extra floor does not.
Exactly how a carport, garage, covered terrace, overhang or outbuilding is treated depends on technical definitions that differ from place to place — and on what is actually registered for the property. This is not something to guess at; it needs to be looked up with the authority that keeps the register.
What you can measure yourself, without guessing, is the outline: walk around the house and measure every side of the facade the whole way round, including every offset, bay window and extension. Note down what is what — house, garage, shed, covered terrace — each separately. That gives you the raw material, and the authority can then decide the classification.
Why the built-up area matters when you want to extend
Built-up area is the figure used when discussing how much of a plot is allowed to be built on. How much that is gets decided locally and can vary from one neighbourhood to the next — no article, and no app, can tell you that. That kind of thing needs approval from the local authorities before work starts, and that is where you ask, before you draw up an extension.
What you can do yourself is get the starting point straight: what is the plot's area, what is the registered built-up area, and what would an extension of, say, 4 × 6 m add on top? 24 m² is 24 m² — the arithmetic itself is simple. It is the rules around it that are not.
When the registered built-up area does not match reality
If an extension was built, a garage torn down, or a shed put up without ever being reported, the old footprint is still sitting in the register. This is one of the most common mismatches, and it turns up exactly where you least want it: when you want to extend.
Get it corrected before you spend money on a project built on a wrong starting point. Your own job is to have the actual measurements in hand — the rest is paperwork with the authority.
How to get your own figures
For the built-up area you need to measure on the outside: walk around the house with a measuring wheel or a long tape, follow every offset, and sketch the outline on paper with a measurement on every side.
For living area and floor space you need to measure on the inside: room by room, length and width at floor level, at two points in each direction. At the same time, measure the exterior wall thickness at three window reveals — that is the piece that ties the two worlds together. Rule of thumb: the exterior walls take up roughly the wall thickness multiplied by the house's perimeter.
HouseSense handles the internal part: scan the home with your iPhone's LiDAR, or draw it on graph paper with your finger. You get a measured floor plan back, with area per room and in total, measured wall to wall, with each storey kept separate — so you can see exactly what sits on top of what.
The app does not produce a built-up area to the register's own definition, and it does not decide what may be built on your plot. But it does give you the house's shape and dimensions, so you can move forward with something measured instead of a guess.
Three figures, three questions — in short
- Built-up area: how much of the plot does the building cover? The footprint, measured on the outside.
- Living area: how big is the home? Every storey's living space, measured on the outside, with internal walls included.
- Your measured floor: how much can I actually stand on, lay flooring on, and furnish? Internal, wall to wall.
Buying materials? The measured floor is what counts. Comparing homes or reading a listing? That is the living area. Discussing what may be built on the plot? That is the built-up area — and there the authorities have the final word.