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Why doesn't the area on my old floor plan match the official record?

Because the drawing usually shows the house as it was planned, with internal dimensions — while the registered area is measured externally. And because houses are rarely built exactly as drawn.

The drawing is a plan, not a measurement

A construction drawing is made before the house stands there. It describes what was intended. During the build a wall shifts 20 cm, a door swaps sides, a window ends up smaller because the ordered one was out of stock — and it rarely gets drawn back in afterwards.

That makes an old floor plan an excellent starting point and a poor proof. It tells you where the rooms sit, how the house was conceived, and where the load-bearing structure is likely to run. It does not tell you how many square metres there are today.

And if the house has been altered since — a wall removed, an attic finished, a garage converted — the drawing describes a house that no longer exists.

The three reasons the numbers don't meet

1. The drawing measures internally, the record externally. The dimension lines on a floor plan usually run from wall face to wall face — the room's internal measurement. The registered floor area is normally measured to the outside face of the external walls, with the internal partition walls counted into the figure. The difference is the walls. Different countries keep this kind of record under different names — the principle is the same everywhere.

2. The house wasn't built exactly as drawn. Deviations of 5-20 cm per wall are entirely normal in older construction. Across a whole house, that adds up to square metres.

3. The drawing may be a different project. Many drawings in a planning authority's archive are application drawings. If the project changed along the way — or was never fully carried out — the original version is still the one on file.

Work out the walls before you call it a mistake

The difference between an external and an internal area can be worked out, and it takes two minutes. The rule of thumb is that the external walls take up roughly the house's perimeter times the wall thickness, and that the internal partition walls take up their combined length times their thickness.

Example. A house measures 10 × 12 m externally and is registered at 120 m². The perimeter is 2 × (10 + 12) = 44 m, and the external walls are 35 cm thick. The walls then take up around 44 × 0.35 ≈ 15 m². Worked out precisely, the internal rectangle is 9.30 × 11.30 = 105.09 m², which is 14.91 m² of wall — the rule of thumb lands within one square metre, because it works from the external perimeter rather than from the wall centreline.

If there are 25 running metres of 12 cm partition walls, they take up 25 × 0.12 = 3 m². Back down to the floor: 105.09 − 3 = roughly 102 m². If the drawing's room areas add up to around that figure, the two sources agree — the 18 m² difference is walls, not error.

You measure the wall thickness yourself at a window reveal, from the internal wall face to the external one. Do it at three windows, because it is rarely the same all the way round.

How to use the drawing correctly

• Use it as a template for where the rooms sit and the shape of the house. That is what an old floor plan is good for.
• Check the dimension lines by measuring two or three rooms with a tape measure. If they match, the drawing is probably reliable for the rest. If they don't, it's historical.
• See what the dimensions run to. Do they run to the wall face or to the wall centreline? That decides whether you need to add half wall thicknesses.
• Note the external wall thickness — many drawings show it in a section. Otherwise, measure it yourself at a window reveal.
• Date the drawing. A sheet with no year on it could predate three renovations.

What to do when the three figures disagree

You typically end up with three numbers: the drawing's, the official record's, and your own tape measure's. Test them like this:

1. Add up the drawing's room areas. That is an internal figure.
2. Add the walls: perimeter times external wall thickness, plus the partition walls' length times their thickness.
3. Compare against the registered floor area.

If you land within 2-3 m², all three sources agree, and the difference was purely the measuring method.

If there is 10-20 m² left over that the walls can't explain, something happened between the drawing and today — an extension, a finished attic, a converted garage — or the record was simply never updated. That is no longer a calculation but a registration question, and it belongs with the authority that maintains the record.

What an old drawing is still best for

An old floor plan isn't worthless just because the area doesn't add up. It's often the only place you can see things that can't be measured today:

• Where the structure runs. A drawing can suggest where load-bearing walls and joists were intended to be. Note: neither the drawing, a scan, nor an app can determine whether a wall is load-bearing today — that requires a chartered surveyor or a structural engineer who has seen the construction.
• Where pipes and stacks were planned. Invaluable if you're thinking of moving a bathroom.
• What was added on, and when. Compare two drawings from different years, and the house tells its own history.
• The terrain and position on the plot. The site plan shows distances to the boundary that are hard to read off today.

So use the paper to understand the house — and a tape measure or a scan to get the numbers.

The easiest route to a number you can trust

If the area is going to be used for anything that costs money — flooring, paint, a contractor's quote, a renovation — measure the house instead of trusting an old sheet of paper.

Measure room by room at floor level, in two places in each direction, because walls in older houses are rarely parallel. Note alcoves and projections separately. Measure the external wall thickness at three windows.

HouseSense turns it into a single workflow: scan the home with the iPhone's LiDAR, or draw it on graph paper with your finger — for instance on top of the old drawing as a reference, with your own measured numbers written in. You get a measured floor plan with area per room and in total, measured internally wall to wall, and you can send it on as a PDF.

The benefit of measuring instead of reusing the paper isn't that the drawing lied. It's that the house has lived since.

The order that saves time

1. Dig out the drawing, and see whether it has a year and dimension lines on it.
2. Measure two or three rooms against it. If they match within a few centimetres, the drawing is usable as a reference.
3. Measure the external wall thickness at three window reveals.
4. Add up the room areas, add the walls, and compare against the registered floor area.
5. If there's an unexplained gap over 10 m², take it up with the authority that maintains the record instead of guessing.