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Should I measure to the wall centreline or the wall surface when two rooms share a wall?

To the wall surface. Measure to the centreline instead, and the two rooms each count half of the shared wall — and the house ends up looking bigger on paper than it really is.

The sum that shows the difference

Two rooms sit side by side with a 10 cm partition wall between them. From the inside of one outer wall to the other is 7.00 m, and both rooms are 4.00 m deep.

Measured to the wall surface — the correct way

• Room A: 3.45 × 4.00 = 13.80 m²
• Room B: 3.45 × 4.00 = 13.80 m²
• Total: 27.60 m²

Check: 3.45 + 0.10 + 3.45 = 7.00 m. The sum works out. The 7.00 m holds two rooms and one wall.

Measured to the wall centreline — the wrong way in this context

• Room A: 3.50 × 4.00 = 14.00 m²
• Room B: 3.50 × 4.00 = 14.00 m²
• Total: 28.00 m²

The difference is 28.00 − 27.60 = 0.40 m². And that isn't a coincidence: the wall's area is 0.10 × 4.00 = 0.40 m². The error is exactly the whole wall — half of it counted into each room.

0.40 m² sounds like little. But a house with 34 running metres of partition wall contains 3.4 m² of wall, and if every room is measured to the centreline, those 3.4 m² show up as floor that doesn't exist. That's enough to shift a price per square metre, and enough to make you order flooring you'll never need.

Why the wall surface is the right answer

A room's area is an answer to the question "how much floor is here?" A wall is not floor. You can't put a table on it, you can't lay parquet on it, and you can't walk on it. So the square metres the wall takes up don't belong to any room.

There is also a practical reason that outweighs the theory: internal measurements are the only measure an ordinary person can check with a tape measure. You can walk into room A, put the tape against one wall, and read it off at the other. You can't measure to the middle of a wall you can't see inside — the centreline is something you have to calculate from an assumed wall thickness. A figure that can't be verified is a worse figure.

So when is the centreline used?

The centreline isn't wrong in itself — it's wrong for room areas. The convention is used when a whole building's floor area has to be divided between several units without losing the walls along the way:

• Flats in an apartment building: a flat's gross floor area is generally worked out to the outside of the outer wall and to the centreline of any wall it shares with a neighbour, so the sum of the flats can add up to the building's total area without the shared wall being left ownerless.
• Gross floor area for a whole building: here every wall counts, because the measurement runs around the outside of the house.

Notice the logic. The centreline is used when a whole building's area has to be divided up and nothing may go missing. The wall surface is used when you want to know how much floor there actually is. The two conventions answer two different questions, and figures from one must never be added to figures from the other.

If you're unsure how your own home is officially recorded, look it up in the public building register where you live. It's the authority behind that register that decides which convention the area is recorded under, and you cannot work that out from your own internal measurements.

How to measure wall thickness without breaking anything open

To check whether a room measurement was taken to the surface or the centreline, you need to know the wall thickness. It can be measured in three places with nothing more than a tape measure:

• In a doorway. Measure the depth of the door's reveal — the distance from one wall surface to the other through the opening. That's the wall thickness read directly. A light partition wall is typically 10-12 cm; an older masonry internal wall, 12-15 cm.
• In a window reveal. The same principle for the outer wall. A cavity wall typically runs 30-38 cm; a newer low-energy house, 40-45 cm.
• At a doorless opening. The depth of the opening is the wall's thickness.

Note that wall thickness isn't the same throughout the house. A load-bearing internal wall might be 15 cm, while a light wall between two rooms is 10 cm. Measure them separately if it matters. And a warning that can't be said clearly enough: a wall's thickness says nothing about whether it's load-bearing. Removing a wall requires a building surveyor or an engineer — it isn't something you or an app can decide from a measurement.

Two rooms must never count the same square metres twice

The rule boils down to one sentence: every square metre of floor in the house belongs to exactly one room, or none. A square metre under a wall belongs to no room. A square metre in the living room belongs to the living room. There is no such thing as a square metre that belongs to two rooms.

That holds in the drawing too, not just in the sum. A shared wall must be one wall between two rooms. Draw two walls sitting up against each other instead — easy to end up with if you draw each room separately — and the house becomes 10 cm wider for every shared wall, and the error carries through to every single area later worked out from the drawing.

The wall's area is the one exception, and it's only an apparent one: a shared wall has two surfaces, and each side belongs to its own room. If both rooms are to be painted, both sides have to be counted. But that's two surfaces, not the same floor counted twice.

How HouseSense handles the shared wall

HouseSense measures and shows rooms in internal measurements — from wall surface to wall surface — precisely because that's the figure you can check yourself with a tape measure.

When two rooms share a wall, it's one piece of wall with two sides, not two walls. When the home is drawn on the app's graph paper, a new wall snaps onto its neighbour's wall and becomes the same wall, so two walls can never end up sitting on top of each other. And when the home is scanned with the iPhone's LiDAR, the rooms' geometry is merged into one connected building before the areas are worked out. Either way, the result is the same: the wall is counted once as wall, zero times as floor — and no square metre can appear in two rooms.