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Can I make a floor plan without remeasuring the whole house?

Yes. If you have an old drawing, a previous scan, or just the house's outer dimensions, three check measurements per floor can replace measuring every single room.

The three check measurements

Whatever the drawing's source, the same three measurements reveal whether it holds up:

1. The largest internal dimension in the house — typically the living room's length or the full length of the hallway. A long measurement exposes small scale errors that a short one hides.
2. A short measurement somewhere else entirely — say, the bathroom's width. If the long one checks out and the short one doesn't, something has been altered somewhere in the house.
3. A wall thickness, measured in a door reveal. This is the figure old drawings most often get wrong, and it's the one that shifts everything inside.

Three measurements take under ten minutes per floor. A full survey of the same floor takes an hour. That's the whole point: you use the old source to skip the 50 measurements, and save the tape measure for the three that decide whether it's true.

How to work out the check

The source says the living room is 4.10 m long. You measure 4.12 m. The difference is 2 cm over 4.10 m, or 0.5 % — that's well within what you'd expect from a drawing, and you can use the rest of the source as it stands.

The source says 4.10 m, but you measure 4.31 m. The difference is 21 cm, or 5 %. Either the source was drawn at a different scale, or the house has been altered. 5 % sounds small, but a room that's 5 % longer than drawn also has 5 % more floor — and if both directions are off the same way, the area error is roughly double that. That's the difference between whether there's room for one more kitchen unit than the drawing shows. In that case, measure the floor yourself.

Work in percentages, not centimetres: 2 cm is nothing in a living room and a lot in a recess.

The sources, best to worst

• The original construction drawings from the authorities' planning archive. Drawn to scale, typically 1:50 or 1:100, with measurements written on them. The best source — but they show the house as it was built, not as it looks after three renovations.
• A previous LiDAR scan. If you've scanned the home before, the geometry is already measured. If you have HouseSense, the home lives in your own iCloud and can be pulled onto a new phone without scanning again.
• Your own old measurements from the last time you painted or laid flooring. Usually correct for the room they were taken in — and useless for the rest.
• The listing's floor plan. Correct on where the rooms sit, rarely correct on how big they are. Often marked "for guidance only" or "not to scale" in the small print.
• The home's official registration. Gives no drawing, but gives areas, storeys and build year as reference points. Remember that official floor areas are almost everywhere measured to the outer face of the outer walls, while you measure on the inside — the two figures aren't meant to match.

The frame method: build inward from the house's outer dimensions

If you have no drawing at all, but can walk around the house, there's a shortcut faster than measuring every room: measure the house's two main outer dimensions, and let the rooms fill the frame.

A house measures 9.60 m × 8.40 m on the outside. The outer wall measures 35 cm in a window reveal. Inside, the house is then 9.60 − 2 × 0.35 = 8.90 m in one direction and 8.40 − 0.70 = 7.70 m in the other.

If two rooms sit side by side across the width, and you've only measured one at 4.12 m, the other can be worked out: 8.90 − 4.12 − 0.11 (the partition wall) = 4.67 m. You've got a measurement you never took — and you can double-check it with a single pull of the tape measure if you want to be sure.

The method works because the sum has to add up. If it doesn't, something is missing: a chimney, a cupboard built into the wall, a service duct. That's a benefit, not a flaw — the maths is telling you there's something you haven't spotted.

How to bring the old plan into an app

In HouseSense, a home can be drawn instead of scanned, and that suits this task well: you already have the shape, you just need to get the numbers in.

1. Drag out the rooms with your finger in the same relative positions as on the old drawing.
2. Type in the measurements you have from the source, in whole centimetres — plus the three check measurements you took yourself, on top. The drawing follows the numbers.
3. Let the shared walls snap together. Two neighbouring rooms that meet share one wall, so the same centimetres are never counted twice.
4. Note in the remarks which measurements were surveyed and which were read off an old drawing. A plan that doesn't say where its numbers came from will sooner or later be used to order materials.

When can't you skip the survey?

• When something has been altered. A removed wall, an enclosed balcony or a conservatory won't exist in the old source. That room needs measuring from scratch.
• When the measurements will be used for a binding quote. A contractor giving a fixed price needs figures that were actually measured — not copied from a drawing.
• When the check measurements differ by more than 1-2 %. Then the source isn't a drawing of your house as it stands today.

And as always: neither the old drawing nor the new plan can determine whether a wall is load-bearing. That takes a building surveyor or an engineer on site.