Why do I get different areas for the same home, depending on who measured it?
Because the public building register, the surveyor, the estate agent, the tradesperson and your own tape measure are each answering a different question. Here is what each figure means, and when to use which.
Short answer: There isn't one single area for a home — there are several, and they don't measure the same thing. The public building register as a rule measures externally, to the outside face of the exterior walls, with the internal partition walls counted in, and the estate agent normally builds on that figure; the surveyor measures according to fixed principles and produces documentation a correction can be built on; the tradesperson measures the surfaces of the job at hand; and you yourself, or a takeoff app, measure internally from wall to wall — the floor. The difference between the external and the internal figure is the walls, on an ordinary detached house often 10-15%.
Five sources, five questions
When five different figures for the same home don't add up, the natural reaction is that someone must be wrong. Usually no one is. The five figures are answers to five different questions.
The public building register: what is recorded about the building?
The registered floor area is a gross area, measured as a rule externally — the line runs along the outside face of the exterior wall all the way round, and the internal partition walls are counted in. Its purpose is to describe the building as a building, for property valuation, insurance, planning administration and statistics. Every country keeps its own version of such a register for much the same purpose. What they all share is that they describe what is registered — not necessarily what actually stands on the plot today.
The estate agent: what can the home be compared to?
The area on a sales listing is normally based on the registered figure, because that is the common ground every price-per-square-metre is calculated on. If the listing differs from the register, there is usually a specific reason: several areas have been added together — home plus basement, say — or a new measurement lies behind it. Ask which.
The surveyor: what is the correct figure under the rules?
A chartered surveyor measures according to fixed principles and delivers documented evidence. That is what the authorities can use if a registered area needs correcting, and it is what a buyer can rely on. It costs money, and the price depends on the size and complexity of the home — get a quote rather than assume a round figure. If you're unsure whether a surveyor is even necessary, ask the authorities first; often less is required.
The tradesperson: how much material, how many hours?
A tradesperson measures what is actually going to be worked on. For flooring that is the floor surface from wall to wall minus fitted cupboards; for painting it is the wall surface, height times length, minus windows and doors. None of those figures resemble the registered floor area, and they aren't meant to. They're meant to resemble the job.
You yourself, and a takeoff app: how much floor is there?
A tape measure, and an app like HouseSense, measure internally from wall to wall, room by room. That is the net area — what you can stand on, lay flooring on and furnish. It is always smaller than the registered figure.
How big is the difference between external and internal?
Rule of thumb: the exterior walls take up roughly the house's perimeter times the wall thickness, and the partition walls take up their combined length times their thickness. A worked example for an ordinary detached house:
- House, external, 10 × 12 m → registered area 120 m², perimeter 44 m.
- Exterior wall 35 cm → 44 × 0.35 ≈ 15 m² of wall. (Precisely: 120 − 9.30 × 11.30 = 14.91 m².)
- Partition walls: 25 running metres × 0.12 m = 3 m².
- 120 m² registered → roughly 102 m² of floor, so 15% less.
You can measure the wall thickness yourself in a window reveal: from the internal wall surface to the external one. Do it at three windows, because it is rarely the same all the way round.
Note that the difference follows the house's shape, not its size. A long, narrow house has a longer perimeter than a square one with the same area, and so loses more square metres to its walls, and a small home loses proportionally more than a large one. That is why there is no single percentage that fits every home — but 10-15% is a usable starting point for a detached house.
When are two areas genuinely at odds?
Test two areas against each other like this: take the external figure, subtract the walls using the rule of thumb, and see whether you land on the internal figure. If you get there within 2-3 m², there is no real disagreement — only two measuring methods.
If there are 10-20 m² left that the walls can't account for, something else is going on: an area under a sloped ceiling, a basement or conservatory filed under the wrong heading, an extension that was never registered, or a registered figure that comes from an old drawing. At that point it isn't a measuring question but a registration question, and it belongs with the authority that keeps the register.
Which figure should you use, and when?
- Property valuation, insurance, listing, price per square metre: the registered area. It is the common yardstick, and it only works because everyone uses it the same way.
- Flooring, paint, tiling, underfloor heating, furniture: your own internal measurement. Order flooring by the registered figure and you will typically buy 10-15% too much.
- Contractor quotes: the area per room, not the home as one single figure. "The house is 120 m²" is not a job anyone can price.
- Correcting the register: documented evidence — ask the authorities what they require before you order anything.
How to get your own figure, once and for all
Measure room by room: length and width at floor level, at two points in each direction, because walls in older homes are rarely parallel. Note alcoves, chimney breasts and projections separately. Add the rooms' areas together — that is your internal floor area. At the same time, measure the exterior wall thickness at three windows, so you can translate to the external figure when someone asks.
HouseSense turns this into one workflow: scan the home with the iPhone's LiDAR, or draw it on graph paper with your finger. You get a measured floor plan with area per room and in total, measured internally from wall to wall, and you can send it on as a PDF to a tradesperson, estate agent or family. Walls shared between two rooms become a single shared wall, so two rooms never count the same square metres twice.
HouseSense does not replace a surveyor and does not decide what counts as floor area under the register's rules. The app gives you the floor — the figure you need for yourself, and the figure that makes the other four make sense.
The short checklist
- The public building register: external gross area, kept by an authority, describes what is registered.
- Estate agent: normally builds on the register — ask if the listing differs.
- Surveyor: documented measurement, the evidence a correction can be built on.
- Tradesperson: measures the surfaces of the job, not the home's area.
- You yourself, and HouseSense: internal floor from wall to wall, room by room.
If two figures disagree, work out which two questions are being answered before you go looking for a mistake. Eight times out of ten, there isn't one.