How do you avoid counting a shared wall twice when adding up rooms?
Measure two rooms separately and add their areas together, and you've typically counted the wall between them twice — once from each room. The error is small per wall, but grows with the number of rooms, and it's easy to avoid once you know where it comes from.
Short answer: A shared wall between two rooms only has one thickness, but it turns up in the measurements of both rooms if you measure the rooms completely independently of each other — and the risk is that it gets counted into both rooms' total area when you add up the figures for the whole property. The fix is to treat a shared wall as one single, shared building element, not two. Digital drawing tools typically solve this automatically by letting rooms that border each other share the same wall line.
Why is double counting a classic mistake?
Measure a house room by room — with a tape measure, a notebook and a calculator afterwards — and the natural workflow is to walk into each room, measure its four walls, and work out that room's area on its own. Do that for every room and add the figures together, and you get a total figure for the whole property. The problem is that every shared wall between two neighbouring rooms turns up in the measurements of BOTH those rooms — and unless you're careful, that wall's thickness can end up being subtracted twice in the total, or in the worse case, not subtracted at all.
The error is rarely big enough to be visible to the naked eye on a single wall. It grows because it repeats for every single shared wall in the house — so a house with ten internal walls can end up with a total area that's clearly wrong, even though each individual room's figure looks reasonable on its own.
What is a shared wall — and why should it only be counted once?
A shared wall (or party wall) is a wall that separates two rooms within the same property — as opposed to an external wall, which separates the property from the outdoors. The shared wall physically has only one thickness and one location in the building, regardless of the fact that it "belongs" to both of the rooms it borders.
When talking about a property's total area, there are two different questions you can ask: "how much floor area is there to walk on in each room" (the net area, without walls) and "how much space does the whole building take up as seen from outside" (the gross area, with all walls). Neither question is answered correctly if a shared wall's thickness is counted twice in the calculation.
How thick is a typical internal wall?
It varies with construction method and age, but roughly: a light plasterboard partition on a thin timber frame is typically around 7-10 cm. An older, brick internal wall can easily be double that or more, and a load-bearing internal wall — whatever the material — is usually thicker than a light, non-load-bearing partition, because it needs to be able to carry a load.
The point of knowing that rough thickness isn't to be able to calculate it in your head, but to be able to recognise when a number looks wrong: if the "shared wall" you've measured is suddenly 25 cm, it's either a load-bearing wall, a chimney inside the wall, or a measuring error — worth checking before the figure is used for anything else.
How do you avoid double counting when measuring manually?
- Measure the whole building's outer edge first, and use that as a control figure. The sum of all the rooms' areas plus all the walls' areas should roughly match the total area within the external walls.
- Note each shared wall as one element with two neighbouring rooms, not as two separate walls — that way it's only in the accounts once, no matter how many rooms you subsequently work out areas for.
- Pick a fixed convention for where a room's measurement "starts and ends". Always measure to the inside of the wall surface (net measurement), and there's no doubt about where the boundary between two rooms lies.
- Check the sum against the whole at the end. If the sum of the rooms is clearly smaller or bigger than what you can measure from outside, there's an error somewhere in the survey.
How do digital drawing tools solve the problem automatically?
In a digital floor plan, where rooms are drawn as geometric shapes, a shared-wall problem can be solved structurally: push two rooms together so they touch, and the tool can let them share the same, common wall line — instead of each room having its own, separate wall that just happens to sit on top of its neighbour's. Then the shared wall only exists in one place in the dataset, no matter how many rooms are having their area read out from it.
That also solves a related problem: colour a shared wall one colour from one room, and it would look different from the neighbour's side if the wall were actually two separate objects. With only one wall line that has two sides, each side can be coloured independently, without one room's choice overwriting the other's.
What does the error mean for the total area if you count it wrong?
A concrete example: a shared wall 10 cm thick and 4 m long occupies 0.4 m². If it's counted twice instead of once, the error is exactly that figure — for one wall. An ordinary house with ten internal walls of that size can therefore easily end up with a total area that's 3-4 m² wrong, purely because the shared walls have been double counted. That's enough to move a house from "under" to "over" a threshold, if the area is used for something where the square metre matters — for example a sale price calculated per square metre.
What HouseSense does
On HouseSense's drawing board, walls snap together when you push one room up against its neighbour: the two rooms then get one shared wall, not two separate ones. Two rooms therefore never count the same square metres twice, and a shared wall can be coloured differently on each side without overwriting the neighbour's choice. That applies both to drawings built up from scratch on the paper and to houses built up from a LiDAR scan.
When should you still check for yourself?
Even with a tool that solves double counting automatically, it's worth knowing the difference between net and gross area when a figure from your own survey needs to be compared with an area from the public building register or a sale listing — they don't always use the same definition of where a room's boundary lies. If the two figures differ significantly, it's rarely an error in the drawing tool itself — it's usually two different questions being answered, and it's worth pinning down which one you actually need.