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Which area is the price per square metre based on?

On the home floor area, not the plot, not the basement, and not the sum of the rooms floors. Mix up two different areas, and the same home can look 45% cheaper without the price changing at all.

The calculation

The price per square metre is a simple division:

Price per square metre = purchase price ÷ floor area

A home priced at 3,250,000 with a floor area of 110 m²:

3,250,000 ÷ 110 = 29,545 per m²

The figures are a worked example, not a market price. The point is the division, not the amount.

The same home, four different prices per square metre

The home in the example — priced at 3,250,000 with a floor area of 110 m² — also has a 60 m² basement that is not approved for habitation, an integrated 12 m² garage, and a total room floor area of 93 m² once the walls are subtracted. Depending on which area you divide by, the answer changes:

• Floor area, 110 m²: 29,545 per m² — the correct, comparable figure.
• Floor area + garage, 122 m²: 26,639 per m² — looks cheaper, but the garage is not living space.
• Floor area + basement, 170 m²: 19,118 per m² — 35% below the correct figure.
• Sum of the rooms' floors, 93 m²: 34,946 per m² — 18% above.

From 19,118 to 34,946 is a swing of 83% upward — or 45% downward the other way. Not a single unit of the price has changed. The entire difference lies in which area was used as the divisor.

Why the floor area is the right choice

The floor area is not "the most accurate" figure — it is the comparable one. Every home for sale states it, it is calculated using the same principles nationwide, and it is recorded in the public building register. A price per square metre is only useful for weighing two homes against each other, and that requires the denominator to be measured the same way in both cases.

The sum of the rooms' floors, by contrast, is the figure you can verify with a tape measure — but that is exactly why it is poorly suited to comparison: no two sellers measure it the same way, and no listing states it. Use it for flooring, paint and furniture; not for the price per square metre.

The five traps

• The basement counts in one home but not the other. The most common mistake. A home with a large basement looks expensive per m², because the basement does not count in the denominator — but the square metres are still there. Compare the price per square metre and ask what lies beyond the floor area.
• The plot's area gets mixed in. A 110 m² house priced at 3,250,000 on an 800 m² plot does not have a price per square metre of 4,063. The plot is part of the price, but it never belongs in the denominator of a home's price per square metre.
• Running costs get forgotten. Two homes with the same price per square metre can differ by a substantial amount each month in running costs. The price per square metre says nothing about the cost of living there.
• The loft floor gets counted in full. Under a sloped ceiling, the lowest part of the floor generally does not count in full towards the floor area — where the cut-off falls depends on the local measurement rules, often set at a fixed ceiling height. A seller who adds in the whole floor makes the denominator too large and the price artificially low.
• Conversions are not registered. If a loft or basement has been converted into living space without being registered, the recorded area will not match reality. Look the building up in the register, and ask the local authority if in doubt — they are the ones who decide the registration.

The price per square metre does not tell you how much home you get

Two homes with a 110 m² floor area at the same price have the same price per square metre — and can still be very different places to live:

• The thickness of the external walls. A new low-energy house with a 45 cm external wall has noticeably less floor than an older house with a 30 cm wall, at the same registered area. You pay the same per registered square metre and get fewer of them to walk on.
• The number of internal walls. Five small rooms cost more square metres in walls than two large rooms.
• The ceiling height. The price per square metre only counts floor. A 3.20 m ceiling gives 28% more air than a 2.50 m one — and costs more to heat.
• The hallway area. A home where 12 m² is hallway has 12 m² less to actually live in than one of the same size with a better layout.

That is why it is worth measuring the home yourself, once you are past the first rough sort. The price per square metre sorts; the tape measure decides.

How to check the denominator yourself

The floor area cannot be measured from the inside, because it is calculated to the outside of the external walls. But the rooms can be measured, and the calculation can be run backwards:

• Measure each room on the inside and add the areas together. Remember the hallway, entrance and utility room — they always get forgotten.
• Add the area of the walls. The external walls' area is roughly wall thickness × the house's outer perimeter, and the internal walls' area is their thickness × their combined length.
• The total should come close to the stated floor area. If the difference is more than 5–7%, there is something worth asking about.

HouseSense gives you the first half of that calculation directly. Once the home has been scanned with the iPhone's LiDAR or drawn on graph paper, each room's internal area and the house's total floor area appear on the drawing — figures you can check yourself with a tape measure. The app does not show a registered floor area, and it should not: the register measures around the house, the app measures inside it. But with both figures in hand, you can see whether they meet where they should.