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Why does a scanned floor sit offset on the floor plan?

When a home has been scanned in several sessions — an extension on its own, a floor on its own — the two parts can land wrongly relative to each other on the drawing. That is not a fault in the scan, but in the stitching, and most apps let you correct it by hand.

Why does this happen at all?

A LiDAR scan keeps track of where the phone is by tracking its movement through the room — from the moment the scan starts. All the geometry it sees along the way is placed relative to that starting point. Start a new scan somewhere else in the house, and it gets its own starting point, and the two scans' coordinate systems have no connection to each other until something or someone tells the app how they relate.

In a house scanned in one unbroken pass, the stitching happens automatically, because the phone never loses tracking along the way. Lose tracking — the battery runs out, the app closes due to an error, you step outside and lose the AR reference — and the next part starts from scratch, leaving you with two islands of geometry that need to be glued together by hand.

When does the problem typically arise?

• A multi-storey house, where each floor is scanned separately because a staircase is too narrow or too steep to keep tracking intact along the way.
• An extension or conservatory, added in a separate scan at a different time.
• An interrupted scan, where you had to stop halfway and resume later.
• Large properties, where the battery or the patience does not stretch to scanning it all in one go.

How do you fix an offset part on the floor plan?

Most apps with this feature ask for two taps: first the wall that needs to move, then the neighbouring wall it should sit against. From those two choices, the app works out the difference in position and rotation, and moves the whole offset part — not just the one wall — into place in a single step. It amounts to telling the app: "this wall here is actually the same physical wall as this one", after which the rest follows.

Is it risky for the measurements to move part of the drawing by hand?

No, provided the move only corrects position and rotation — not size. A well-built implementation moves and rotates the offset part as one rigid whole, without stretching or squeezing the geometry. Each room's own measurements, captured during the scan itself, do not change; it is only the room's position relative to the rest of the house that gets corrected.

Can you undo it if you tap the wrong wall?

In a well-built feature, yes — this is precisely a place where mis-clicks happen easily, because you have to hit the right wall among several identical-looking lines on a drawing. An undo option is not a nice-to-have here; it is a precondition for people daring to use the feature at all, without being afraid of ruining a scan they spent a long time capturing.

What if there is no shared wall to click on at all?

Then geometric stitching cannot happen automatically — for example if an extension is connected to the main house through an outdoor passage that was not itself scanned. Here you have to place the part manually, using a measurement you took yourself with a tape measure, or from an aerial photo of the plot. It is a reminder that even an advanced scan is, at bottom, geometry without context, until a human tells it how the parts physically fit together.

Is it better to just try to scan everything in one go?

Yes, whenever possible. A single, unbroken pass through the whole home saves you from having to correct offsets afterwards and removes a source of error entirely. But it is not always practically possible — battery life, the size of the house, a staircase that disrupts tracking, or simply being interrupted by an occupant, a phone call, or a door that will not open. A good app therefore needs to handle both situations, not just the ideal one.

A concrete example: an extension scanned a week later

Imagine the main house was scanned in one go on a Sunday, while a new conservatory was not scanned until a week later, once the builders had finished. The two scans share no common starting point — the app only knows there are two groups of rooms, not how they face each other. Without a manual stitch, the conservatory on the drawing could land anywhere: offset two metres to one side, rotated 15 degrees, or in the worst case mirrored relative to reality. Tap the shared wall between the living room and the conservatory in both scans, though — the same physical wall, seen from each side — and the app corrects the difference in one step: the conservatory rotates and shifts so the wall lands exactly on top of the corresponding wall from the main house. It is the same calculation whether the offset is ten centimetres or several metres; only the precision of your tap decides how neat the result turns out.

What HouseSense does

HouseSense has a feature in the floor plan's menu for exactly this: two taps — the offset wall, and the neighbouring wall it should sit against — move and rotate the whole offset part into place, with undo if you tap the wrong one. It requires the house to be editable (not a house you only have shared viewing access to). The rooms' own measurements do not change; only their position and rotation relative to the rest of the house are corrected.

How to avoid and fix offset scans

1. Plan the route, so you can cover as much as possible in one unbroken pass, before the battery or the patience runs out.
2. Spot the problem early. Check the floor plan while the house is still empty and easy to rescan, not the week after you have moved in.
3. Pick two unambiguous walls for a manual stitch — corners and walls with a window are easier to tap correctly than a long, uniform surface.
4. Measure it in reality if the automatic stitch still looks wrong, before trusting the final drawing.