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Can I make a floor plan from an old estate agent listing?

Yes, but only as a starting point: the listing's floor plan correctly shows which rooms exist and where they sit — the measurements you need to get from somewhere else.

What the listing's floor plan is good for

A listing from an estate agent almost always includes a small floor plan per storey. It's drawn from the building file or measured quickly for the sale, and its strength is the layout: how many rooms there are, which way the doors swing, where the kitchen sits relative to the living room, where the stairs are, and which side of the house the windows face.

That's actually the hard half of the job when redrawing a home. Remembering that there's a recess behind the utility-room door, or that the hallway has a bend in it, is exactly what people forget when drawing from memory. With the listing in hand, you don't have to remember anything — you just have to measure.

Why the measurements usually don't hold up

There are three reasons the square metres in a listing rarely translate directly:

• The drawing is often not to scale. Read the small print: if it says "indicative floor plan", "not to scale" or "for illustration only", it was never meant to be measured from. Rooms may be stretched so the label would fit.
• The floor area is calculated on the outside. The figure quoted in a listing usually comes from the official property register, and official floor areas are almost everywhere measured to the outer face of the outer walls. You live on the inside. If the house has 35 cm outer walls, the walls eat up 10-15 % of the outside area — less in a large house, because the walls follow the perimeter while the area grows faster.
• The home may have been altered. A listing from 2011 knows nothing of the wall that came down in 2018, or the conservatory that was added. Everything since is missing.

So: use the drawing for what and where — get how big from a tape measure.

The method: five measurements make an old plan accurate

You don't need to measure every single wall. You need enough figures to lock the drawing in place and to reveal whether the scale even holds.

1. The house's two main outer dimensions — length and width, measured along the façade. Those two figures set the frame for the whole house.
2. One room's two internal sides — ideally the largest room, such as the living room, measured corner to corner at floor height.
3. The outer wall's thickness, measured in a window or door reveal. Typically 25-40 cm in an older brick house.
4. The partition walls' thickness, measured in an internal door reveal. Typically 10-12 cm for a lightweight wall.
5. A diagonal across the large room, which reveals whether the house is square.

With those five figures you can check the scale: measure the living room's width on the paper with a ruler, and hold it up against reality. If the living room is 4.12 m in real life and 8.2 cm on paper, the drawing is at 1:50 (8.2 cm × 50 = 4.10 m — close enough to hold up). If you instead measure 6.9 cm on the paper, the scale doesn't add up, and you can't read off the other figures — you'll have to measure them yourself.

The diagonal check, worked out

A room measures 4.12 m × 3.05 m. If it's square, the diagonal should be the square root of (4.12² + 3.05²) = square root of (16.97 + 9.30) = square root of 26.27 = 5.13 m. If you measure 5.21 m, the room is out of square by about 8 cm on the diagonal — and you'll never see that on a listing's floor plan, where every corner is drawn as a clean 90 degrees.

The room's area is 4.12 × 3.05 = 12.57 m². Compare that with what the listing states for the same room. If the difference is more than a couple of per cent, you've found out how much you can trust the rest of the drawing.

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 a plan, you just need real numbers on it.

1. Put the listing beside you, and drag out the rooms with your finger in the same order and relative position as on paper.
2. Type in the measurements you've taken, in whole centimetres. The drawing follows the numbers — not the other way round.
3. Let rooms you haven't measured inherit their width from their neighbour: a hallway is usually exactly as wide as the house is deep minus the rooms on either side. Enter that figure, and spot-check it in one place with the tape measure.
4. Place doors and windows where the listing shows them. Measure the width of one window of each type; the rest are usually identical in a house from the same build year.

Every measurement in HouseSense is worked out internally — wall to wall, exactly how you measure it yourself — and walls between two rooms snap together into one shared wall, so the same centimetres are never counted in both rooms. That's the mistake that otherwise makes a home-drawn floor plan grow by a couple of square metres compared with reality.

If the listing is your only source

If you've sold the house, or haven't been inside the home yet, you can't measure anything. The plan can still be used — but it has to be labelled honestly. Note in your own drawing, or in the report's remarks, that the measurements are estimated from a listing and not measured on site. A floor plan that doesn't say where its numbers came from will sooner or later end up being used to order flooring.

Two other sources are often better than the listing:

• The authorities' archive of the building's planning file. The original construction drawings are drawn to scale, typically 1:50 or 1:100, with measurements written on them. Many places let you view the archive online.
• The home's official registration. It doesn't give you a drawing, but it gives you areas, build year, number of storeys and rooms — useful reference points when checking whether your own measurements land somewhere sensible.

And one thing no old plan can tell you

Neither the listing, the construction drawing nor your new floor plan can determine whether a wall is load-bearing. Lines on paper say nothing about what a wall holds up. If a wall is coming down, a building surveyor or an engineer has to assess it on site.