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What does a scan catch that I would overlook myself?

Piers between windows, alcoves, chimney breasts, crooked walls, and doors sitting somewhere other than you remember. A scan has no expectation of what a room ought to look like — and that is its biggest advantage over your own drawing.

The brain corrects before you notice

People are poor witnesses to geometry. We remember rooms as rectangles, and we remember furniture better than walls. Ask ten people to draw their own living room from memory, and most will produce a rectangle with a door in the middle of one wall — even when the living room has a chimney breast, a 40 cm projection beside it, and a door sitting 60 cm from the corner.

A LiDAR scan has no expectations. It measures the distance to everything it sees, and it does not know that rooms "usually" are rectangular. That is the entire advantage: it cannot straighten reality out to fit an assumption, because it has none.

The ten things people most often miss — and a scan does not

• Piers between windows. Two windows with 30 cm of wall between them are often drawn as one long window. That changes both the window area and where a cupboard can stand.
• The alcove behind the door. Many rooms have a recess where the door swings open into. It is 15–30 cm deep and is always forgotten, because the door is standing right there.
• Chimney breasts. A chimney breast typically projects 25–50 cm into the room and runs all the way to the ceiling. It is easy to miss when a bookcase stands in front of it, and expensive to miss when the kitchen is being laid out.
• Pipe boxing and soil stacks. Especially in bathrooms and kitchens. A 30 × 30 cm box in a corner means the tiles do not run all the way round.
• Dropped ceilings. A ceiling lowered by 20 cm at one end to hide ventilation ducting changes the room volume and, with it, the heating calculation.
• Crooked walls. A 5–8 cm difference from one end to the other is entirely normal in older houses and completely invisible to the eye.
• The door's true position. Not "in the middle", but 62 cm from the corner. That decides whether the bed can go there.
• The window sill height. How high above the floor a window starts. That decides whether a kitchen worktop would run into it.
• The angles between rooms. That the hallway does not run exactly perpendicular to the living room. You draw it square because you walk through it every day without thinking about it.
• Ceiling height that is not consistent. Old houses often have different ceiling heights from room to room, because the floor build-ups were made at different times.

The sum: what an overlooked projection costs

You draw a living room as 4.50 × 5.20 m: 23.40 m². In reality there is a chimney breast of 1.20 × 0.45 m at one end. It takes up 1.20 × 0.45 = 0.54 m², so the real floor area is 22.86 m².

Half a square metre sounds like nothing, and for paint and heating it is nothing. But if you are laying plank flooring with boards running wall to wall, every board at the chimney breast has to be cut around a corner — and if you order a 3.20 m sofa for exactly that wall, you only have 4.50 − 1.20 = 3.30 m clear. That fits, but with 5 cm to spare on each side. Had the chimney breast been 1.40 m wide, it would not have fitted.

That is how an overlooked projection does damage: not in the square-metre figure, but in everything that has to fit.

What a scan does not catch

Scepticism runs both ways. A LiDAR scan is not an X-ray machine, and it sees neither behind walls nor into constructions. It does not catch:

• Whether a wall is load-bearing. Neither a scan nor any app can determine that. It takes a building surveyor or an engineer looking at the construction — not at a floor plan.
• What the wall is made of. Plasterboard, aerated concrete, brick and timber frame all look the same from the outside.
• Pipes, cables and ventilation inside the wall.
• The wall behind the cupboard. The sensor measures the cupboard front. If a 60 cm built-in wardrobe runs the length of the wall, that is what gets measured, unless you open it up.
• The wall behind the curtain. Same problem, just softer.
• Glass. Light pulses pass through panes and bounce off mirrors. Large glazed areas and mirrored walls are the scan's weakest point.

That is why the best takeoff is a scan plus five minutes of checking: pull the curtains back, open the cupboard doors, and take one control measurement with a tape measure in every room that will be used for anything that costs money.

What you do with what the scan found

When the scan has found an alcove or a pier you did not know about, it is not just a curiosity. It shifts three things:

The area. HouseSense calculates the area of the room's actual shape, not the rectangle you thought it was. Alcoves add to it, projections subtract from it.

The takeoff for the contractor. Wall areas, floor areas and ceilings are all calculated from the same geometry. A pier adds two extra corners to paint and more metres of skirting.

The 3D model and the sunlight. A window's true width and position decide where the sun falls in the room, and when. If the window is drawn 40 cm out of place, the sunlight is also 40 cm out of place.

So should I always scan instead of draw?

No. The scan wins on one point: it finds what you did not know was there. It only does that if you are in the home with an iPhone or iPad that has LiDAR, and if you can move along the walls.

If you are not in the home, drawing is not a worse choice — it is the only choice. And a drawing becomes good in exactly the way a scan is good: by you walking around with a tape measure and writing down the figures you would otherwise have guessed. If you draw it yourself, put in precisely the alcoves, piers and projections you have measured — those are what decide whether things fit.

The honest way to put it is: the scan catches what you would overlook, but only in the rooms you actually scan. Everything else, you have to check for yourself.