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Why doesn't the official floor area match what I measure myself?

Because official areas are usually measured from the outside, to the outer face of the exterior walls, while you measure from the inside, wall to wall. The difference is the walls themselves, and it is bigger than most people think.

The two figures aren't measuring the same thing

It isn't a mistake that the registered area and your own measurement disagree. They are two different quantities serving two different purposes.

The official area is a gross area. It is usually measured from the outside: the line runs along the outer face of the exterior wall all the way round. Everything inside that line counts — including the internal partition walls, the masonry wall of the bathroom, and the chimney. Its purpose is to describe the building as a building: valuation, insurance, planning records, statistics. Different countries keep this kind of register under different names and different rules for what counts, but the principle is the same everywhere.

Your own measurement is a net area. You put the tape against one wall and pull it to the other. What you're measuring is the floor — the floor you'll buy boards for, paint walls around, and put a sofa on. Walls don't appear in that figure, because you can't stand on them.

Both figures are correct. They simply answer different questions.

The maths: where do the square metres go?

Take a rectangular house that measures 8.00 × 10.00 metres on the outside. The registered area is 80 m².

The exterior wall is 35 cm thick — a perfectly ordinary figure for a house from the 1970s with added insulation. That thickness has to come off both ends of both directions:

• 8.00 m − 0.35 m − 0.35 m = 7.30 m
• 10.00 m − 0.35 m − 0.35 m = 9.30 m
• 7.30 × 9.30 = 67.9 m² internally, if the house were one big room

So the exterior walls alone have eaten 12.1 m² — 15% of the outside area — before a single partition wall has even gone up.

Then come the partition walls. Say the house has four internal walls totalling 22 running metres, each 12 cm thick: 22 × 0.12 = 2.6 m². That leaves 65.3 m² of floor to be measured out room by room. Add up the individual rooms and that is where you'll land — not 80.

The rule of thumb: walls cost you perimeter, not area

Here is the rule worth remembering: the area the exterior walls take up is roughly the wall thickness multiplied by the house's perimeter.

The house above has a perimeter of 2 × (8 + 10) = 36 metres. 36 × 0.35 = 12.6 m² — almost exactly the 12.1 m² we worked out above (the small gap is the four corners, which the rule counts twice).

The consequence isn't intuitive: the gap between the outside and the inside area follows the house's shape, not its size. Two houses both 80 m² on the outside can lose very different amounts:

• A square house, 8.95 × 8.95 m: perimeter 35.8 m → about 12.5 m² in walls.
• A long, narrow house, 5.00 × 16.00 m: perimeter 42 m → about 14.7 m² in walls.
• An L-shaped house with the same area has an even longer perimeter and loses even more.

Same wall thickness, same outside area — over 2 m² of difference on the inside. That's why "what percentage should I deduct?" doesn't have a single answer. Measure the perimeter and multiply by the wall thickness; it takes two minutes and beats any percentage.

Finding your wall thickness without drilling a hole

You can read the exterior wall's thickness off any window or exterior door. Measure from the inner face of the wall to the outer face — that's the depth of the reveal plus the frame. Do it at two or three different windows, because the thickness is rarely the same all the way round a house that's been extended or had insulation added.

The rough size follows the age of the house. Old houses with solid masonry or an uninsulated cavity wall are thinnest. Houses from the middle of the last century with an insulated cavity wall run noticeably thicker. Newer houses, built to stricter insulation rules, have the thickest exterior walls — in some cases more than double the old ones. Partition walls are considerably thinner than exterior walls, and a masonry internal wall is thicker than a light stud-and-plasterboard one.

The figure you measure yourself in your own window reveal always beats a table.

Four other reasons the numbers don't meet

1. Sloped ceilings and knee walls. In a one-and-a-half-storey home, or under a sloped roof, there is floor you can't stand upright on. There are technical rules for when and how the area under a sloped ceiling counts towards the official area, and they aren't something to guess at. If your house is one-and-a-half storeys, this is almost always where the big unexplained difference is hiding — look it up with the relevant authority rather than working it out yourself.

2. The register can simply be wrong. Many register entries come from old paper drawings or a survey nobody alive today has seen. If an extension was built, a garage was converted, or a terrace was enclosed since the last registration, the old figure can still be the one on file. The register doesn't correct itself.

3. You might not be measuring what you think. Did you measure to the skirting instead of the wall? Did you deduct the alcove behind the wardrobe? Are the walls even parallel? In older houses they rarely are — measure the same room at both ends and note both figures.

4. It isn't the same area at all. Living area, footprint, total building area and basement area are four different fields in the register. Hold your floor measurement up against the wrong field and the gap can look enormous. Check what the field is actually called before you do the sums.

Which figure should you use, and when?

Use the official area when talking to the outside world: valuation, insurance sums, sales material, loan applications, price-per-square-metre comparisons with other homes. It's the shared yardstick, and it's only useful because everyone uses it the same way.

Use your internal measurement when you're buying something: flooring, paint, underfloor heating, tiles, carpet, a new sofa. Order flooring for 120 m² because that's the registered figure, and you'll end up with 15 m² too much stacked on pallets in the driveway. That's an expensive rounding error.

And use the room-by-room figures when you ask for a contractor's quote. "The house is 120 m²" isn't a job anyone can price. "Living room 28.4 m², kitchen 14.1 m², hallway 9.8 m² — all floors to be replaced" is.

Getting your own figure measured properly

Measure room by room, not the house as a whole. In each room: measure the length and width at floor level, measure both directions in two places, and write down both figures if they differ. Note alcoves, chimney breasts and projections separately and deduct them. Add up the rooms' areas — that's your internal floor area.

HouseSense always works internally, wall to wall, whether the home is scanned with the iPhone's LiDAR or drawn on graph paper with a finger. The app gives you the area per room and in total, and because walls shared between two rooms become a single wall, two rooms never count the same square metres twice.

What HouseSense can't do is tell you whether the registered area is correct. No app can. What the app gives you is a measured internal figure you can hold up against the register — and a drawing to point to if the two figures need explaining to someone else.

When is the difference too big?

Work out the walls with the rule of thumb — wall thickness times perimeter — add a couple of square metres for the partition walls, and see whether that accounts for the gap. If it does, everything is exactly as it should be.

If 10, 15 or 20 m² are left over that the walls can't explain, something else is going on: a wrongly registered figure, area under a sloped ceiling, an extension that was never registered, or a basement or conservatory that's ended up in the wrong field. That's when it's worth pulling up the register's own records and reading what's actually listed — and, if in doubt, asking the authority that keeps the register. It isn't something to work out on your own.