Published 2026-10-29 · 2 min read
How to put an old floor plan under your model and scale it
You have the house plan on paper, from the builder or the deeds, and you want to see it in 3D. You do not need to measure everything again with a tape: put the plan under the model, bring it to scale and draw the walls right over it. It takes a quarter of an hour.
1. A straight image
- Best: a scanner. If you photograph it, hold the phone parallel to the sheet, right above it, and use a scanning app that corrects perspective.
- Crop the image as close as possible to the edge of the drawing.
- Even light, with no phone shadow over the plan.
2. Work out the image's real width
The editor asks for one number: how many metres the full width of the image represents. You get it from a known dimension.
- Open the image in any program that shows pixel coordinates.
- Pick a long, reliable dimension, say an 8.40 m external wall. Measure its length in pixels: say 1,800 px.
- The image is 3,000 px wide. Real width: 3,000 × 8.40 / 1,800 = 14.0 m.
- The longer the reference dimension, the smaller the error.
3. Place it and calibrate
- In the architecture editor, in the Plan, dimensions and costs panel, press “Add a plan underlay” and choose the image. It lies on the floor, semi-transparent.
- Press “Calibrate the underlay” and type the width you calculated: 14.
- Check: with the Measure tool, measure another known dimension on the plan, in the other direction. If it is off by more than 1%, the scan is skewed — take the picture again.
4. Draw over it
Place the house or rooms and match them to the walls on the plan, then doors and windows in the drawn openings. When the model is done you can hide the background plan from the scene list, with the eye next to its name.
Old paper plans shrink or stretch over time, and the written dimensions are more reliable than the drawing. When drawing and dimension disagree, trust the dimension.
Does it work with a hand sketch?
Yes, if it has at least one written dimension. The result is only as accurate as the sketch — fine for an idea, not for construction.
Can I stack one floor's plan over another?
The editor holds one background plan at a time. Do the ground floor, then replace the image with the upper floor's plan and calibrate it the same way.
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