My scan and my own drawing give different numbers — which do I trust?
Trust the tape measure you hold in your own hand, at the right spot. Here are the five reasons a scan and a drawing of the same room don't agree — and how to work out who's right.
Short answer: a tape measure you hold in your own hand at the right height beats both. If the difference is under a couple of centimetres, it has no practical effect — use the scan's figure. If it's bigger, it's almost always one of five things: you measured at different heights, the scan hit a piece of furniture instead of the wall, the room is out of square and you each measured a different end, one figure is external and the other internal, or the skirting board got included in one of the measurements.
First question: how big is the difference?
Sort the difference by size before you go looking for a cause. It saves time, and it decides whether there's even a problem.
Under 2 cm: no practical effect on areas, paint, flooring or heating. On a 3.60 m room, 2 cm is half a percent. Leave it, unless something has to fit precisely.
2-10 cm: a real disagreement. Usually down to measuring technique: different heights, a skirting board, or an out-of-square room measured at each end. Find the cause — it's usually easy to spot.
Over 10 cm: something else entirely is wrong. Either the scan measured a piece of furniture or a surface that isn't the wall, or you're comparing external and internal measurements. Measure again on site.
The five most common causes
1. You measured at different heights
Real walls aren't vertical. Old masonry can easily lean a couple of centimetres from floor to ceiling, and a room that's been retrofitted with insulation often has a thicker wall at the bottom. If you measure at the skirting board and the scan takes its reading at chest height, you get two different — and both correct — figures.
The fix: always measure at about 1 metre height. That's the height furniture and kitchen units meet the wall at, and it's the one you should use when ordering something.
2. The scan hit something that wasn't the wall
LiDAR measures the nearest surface it can see. If there's a skirting board 2 cm deep, a curtain 5 cm deep, a radiator 10 cm deep or a built-in wardrobe 60 cm deep in front of the wall, that's what gets measured. The room comes out smaller than it really is.
The fix: look at the floor plan to see if a wall sits suspiciously close to something. Pull the curtain back, open the wardrobe, and scan the room again — or correct the wall's position on the drawing board using your own measurement.
3. The room is out of square, and you each measured a different end
A room that's 3.62 m at one end and 3.68 m at the other gives a 6 cm disagreement without anyone having measured wrong. It's the most common explanation in older houses.
The fix: measure both ends. If they differ, you've found your answer — and at the same time you've got the one figure that actually matters when a cupboard has to go in along the wall: the smaller measurement.
4. One figure is external, the other internal
HouseSense works in internal measurements, that is, from wall surface to wall surface. The official property register — the public building register, in most countries — and most estate agent listings state the area externally, including the outer walls. That holds in most countries: the register measures the house from outside, you measure the room from inside. If you drew from a listing and scanned reality, you're comparing two different things.
The arithmetic: a house that's 8.00 × 10.00 m externally gives 80 m². If the outer wall is 35 cm, the internal measurement becomes 8.00 − 0.70 = 7.30 m and 10.00 − 0.70 = 9.30 m, so 7.30 × 9.30 = 67.9 m². A good 12 m² difference — and the internal walls haven't taken their share yet. Both figures are correct; they just don't measure the same thing. If you're unsure what officially applies to your home, the property register has the answer — not your tape measure.
5. The skirting board got included in one measurement
A skirting board is typically 1-2 cm deep. If you measure along the floor from skirting to skirting, you get 2-4 cm less than the wall's real internal measurement. For flooring, that's actually the figure you want — the floor has to go under or up against the skirting anyway. For paint and for cupboards, it's the wrong one.
The fix: be deliberate about which figure you're using for what, and note it down.
How to settle it in two minutes
- Go into the room with a tape measure.
- Measure at about 1 metre height, from wall surface to wall surface, no skirting board.
- Measure both ends of the room. Write down both figures.
- Compare with the scan's figure and with your drawing.
- If the scan is closest: keep it. If your drawing is closest: correct the scan on the drawing board.
You don't need to rescan the room to fix one figure. You can move the wall on the drawing board and type in the measurement you took yourself. The rest of the home stays as it is.
What NOT to do
Don't add the figures together and take the average. An average of a correct and an incorrect measurement is still incorrect — it just looks more considered. Work out which one is right.
Don't pick the figure that best matches the official area. The register measures something else — the house from outside. Forcing your internal figure up to match the register's area makes it useless for everything you actually need it for.
Don't smooth away the crookedness because it looks messy. The crookedness is often the most important piece of information in the room. A floor plan that has straightened away 6 cm to look tidy is a drawing that lies — and it's precisely those 6 cm that decide whether the cupboard fits.
When the difference matters — and when it doesn't
Doesn't matter: paint, floor area, heating calculations, a listing's approximate size. Here a couple of centimetres are lost in the margin you'd have to allow for anyway.
Matters completely: anything that has to fit between two walls. Kitchen units, built-in cupboards, a sofa for an alcove, a worktop, a sliding door. Here you need the smaller figure you've measured, at the height the item will stand, and you need to have measured it yourself.
The rule behind it all stays the same throughout: a floor plan may be tidy, but the numbers have to be true. If in doubt, measure on site — it takes two minutes and is cheaper than a worktop that's 3 cm too long.