Why is a scanned room never perfectly square?
Because the room isn't square. A LiDAR scan measures the walls exactly as they stand — and real walls are crooked, curved and unevenly plastered. Here is what that unevenness is made of, and when you should straighten it out.
Short answer: a scanned room doesn't come out square because the room itself isn't square. Walls are rarely quite parallel, corners are rarely exactly 90°, and plaster, skirting and built-in cupboards make the surface uneven. On top of that comes the scan's own uncertainty. So the unevenness is really two things mixed together: the room's genuine irregularity, and the measurement's noise — and they need to be treated differently. The room's own irregularity should be left standing. The noise is fair to correct.
The unevenness is two different things
When a LiDAR scan gives you a room where one wall measures 3.62 m and the opposite one 3.68 m, there are two possible explanations, and they aren't equally good:
1. The room really is uneven. This is the most common answer, especially in homes built before 1970 and in anything altered along the way. Bricklayers worked to a string line and by eye, floors have settled, and an extension rarely meets the old house at exactly 90°.
2. The scan got it wrong. The sensor saw the curtain instead of the wall behind it, you walked past a corner too fast, or the room was so dark and reflective that the reading became unreliable.
The difference matters completely. Straighten out the room's genuine unevenness, and you have a prettier drawing and a wrong number. Leave the scan's noise standing, and you have a drawing that looks careful and is just as wrong. The rule at the heart of HouseSense is that the drawing may be tidied up, but the numbers may not.
What makes a real room uneven
- Age. Older brickwork was laid to a string line, not a laser. Deviations of a few centimetres over the length of a wall are entirely normal and don't mean anything is wrong.
- Settling. A house keeps settling for decades after it's built, and it doesn't settle evenly in every corner.
- Plaster and added insulation. A layer of plaster is never quite the same thickness across a whole wall, and internal insulation makes a room smaller — often not by the same amount at both ends.
- Renovations. A demolished wall leaves a trace, a new stud wall sits wherever there happened to be room, and a dropped ceiling doesn't necessarily follow the old geometry.
- Skirting, radiators and built-in cupboards. They aren't the room's walls, but they're what the sensor sees first.
Some rooms genuinely are square: new builds, precast concrete, plasterboard put up with a laser. Scan a newly built room and it comes out closer to right-angled precisely because it is right-angled. The scan hasn't got better — the room has.
What makes a scan unreliable
LiDAR measures distance by sending out short pulses of light and timing how long they take to come back. That works well on matte, solid surfaces and worse on five things:
- Glass and mirrors. The light pulse passes straight through the pane or bounces off somewhere else. A large mirror can make a room look as if it keeps going.
- Glossy, dark surfaces. High-gloss black returns very little light.
- Soft things. Curtains, sofas and clothing aren't walls, but they stand in front of the walls and get measured instead.
- Distance. The farther away a surface is, the more the points scatter. Large rooms are measured best by walking closer to each wall.
- Speed. Walk too fast and the sensor gets fewer points per surface, leaving the software less to work with.
All of this can be fixed: pull the curtains back, walk at a steady pace, walk all the way round along the walls instead of standing in the middle and spinning on the spot, and take an extra pass through a room with a lot of glass.
What the app does to the unevenness — and what it doesn't
HouseSense straightens a scan out wherever there's good reason to: walls that clearly belong together get aligned with each other, and near-right corners are allowed to become right angles. That's what makes the floor plan readable, and it's what's meant by the drawing being allowed to be tidied up.
But the correction has a limit, and that's deliberate. A wall that's noticeably out of true stays out of true, because it is out of true. A floor plan that has straightened away 9 cm just to look neat is a drawing that lies — and those exact 9 cm are what decide whether the cupboard fits into the alcove.
So: if you're unsure of a number, measure it with a tape. One check measurement in the room it concerns is worth more than any assumption about how good a sensor is.
When to straighten it out, and when to leave it
Straighten it out when you can see the scan got it wrong: a wall that bulges where you know it's straight; a corner that's been rounded off because a bookcase stood there; a door that has landed 40 cm from where it actually sits. In HouseSense you can correct the geometry on the drawing board afterwards — move a wall, place an opening to the right measurement, draw in the piece the scanner never saw — without rescanning the whole floor.
Leave it when the difference is small and consistent: 3.62 and 3.68 m at either end of an old room is probably the room's true shape. Round it to 3.65 m at both ends and you've thrown away the one piece of information actually worth having — namely that the room is uneven.
The sums: what the unevenness costs in practice
A room measures 3.62 m at one end and 3.68 m at the other, and is 4.50 m deep. Treat it as a rectangle of 3.65 × 4.50 m and you get 16.43 m². Work it out as the trapezoid it actually is, and you get the same area: (3.62 + 3.68) ÷ 2 = 3.65 m average width, so 16.43 m² again. For flooring and paint, the unevenness makes practically no difference at all.
But if you need a cupboard or a run of kitchen units 3.65 m long along the wall where the room is only 3.62 m, it won't fit. For areas, averaging is fine. For something that has to fit, use the smallest measurement — and measure it yourself, on site, at the height the thing will actually stand.
So is a drawing more accurate than a scan?
No — it's just wrong in a different way. A drawn home is square by definition if you draw it square, and you've removed the unevenness before it was ever measured at all. The drawing looks more certain precisely because it shows no doubt.
That isn't an argument against drawing. It's an argument for drawing from a tape measure and writing in both figures if the room is clearly uneven. If you choose to draw an uneven room as a rectangle, make that choice deliberately, and note the smallest measurement where it matters.
The honest ranking is this: a tape measure in hand, in the right spot, beats everything. A scan comes second and, in return, catches what you wouldn't have noticed. A drawing from memory is the riskiest of the three, however neat it turns out.
Five things that will make your next scan better
- Walk along the walls instead of standing in the middle of the room and turning.
- Pull curtains back, close cupboard doors, and move whatever can be moved away from the wall.
- Turn the lights on. LiDAR works in the dark, but the software also uses the camera image to understand what it's seeing.
- Hold the phone at roughly chest height and angled slightly down, so both the floor line and the wall surface are captured.
- Take one check measurement with a tape in every room you're going to order something for.