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How do I draw an L-shaped room when I can't scan it?

Split the room into rectangles, measure six figures instead of four — and let the sum of the parts prove the measurements are right.

The split: two rectangles instead of one L

An L-shaped room is two rectangles pushed together. Stand in the room and extend the short wall — the one that forms the inward-facing corner — all the way across to the opposite wall. That imaginary line splits the room into two rectangles, and rectangles you can measure.

There are always two ways to split an L, and they give the same area. Pick the one where both parts end up as close to square as possible — those are easiest to measure and easiest to draw.

The six measurements you need

1. The full length of the long external wall — the entire length of one leg of the L, corner to corner.
2. The short side at the same end — the room's width, where it's widest.
3. The length of the short leg — from the inward-facing corner out to the end.
4. The width of the short leg.
5. The diagonal of the large rectangle.
6. The diagonal of the small rectangle.

The two diagonals are the ones everyone skips, and they're the ones that reveal whether the corners are actually 90 degrees. In an older house they often aren't.

The sums, written out

An L-shaped room: the large rectangle measures 4.12 m × 3.05 m, and the short leg measures 2.20 m × 1.60 m.

Area: 4.12 × 3.05 = 12.57 m². 2.20 × 1.60 = 3.52 m². Total 16.09 m².

Diagonal check, large rectangle: the square root of (4.12² + 3.05²) = the square root of (16.97 + 9.30) = 5.13 m. If you measure 5.21 m, that part of the room is out of square.

Diagonal check, short leg: the square root of (2.20² + 1.60²) = the square root of (4.84 + 2.56) = the square root of 7.40 = 2.72 m.

The check measurement that ties it all together: measure the room's full external length along the wall where the two parts meet. It should come to 3.05 + 1.60 = 4.65 m, if the legs are split across that direction. If it doesn't add up, you've measured one of the six figures wrong — and you caught the mistake before it went onto the drawing.

How to draw it up

On paper at 1:50 (1 small millimetre square = 5 cm) you draw the two rectangles side by side and rub out the shared line. 4.12 m becomes 8.24 cm, 3.05 m becomes 6.1 cm, 2.20 m becomes 4.4 cm and 1.60 m becomes 3.2 cm.

In an app you'd typically draw the room as a rectangle and then drag one corner in, or draw two rooms and let them merge. In HouseSense the home is drawn on digital graph paper, where the measurements are typed in as whole centimetres — the drawing follows the figures, not your finger. That's decisive with an L: your finger's precision would never hit 1.60 m on the short leg, but a typed figure hits it every time.

The same rules for bay windows, niches and chimneys

Anything that juts in or out of a room is handled the same way: as a small rectangle that's added or subtracted.

• A niche (behind a door, say, or in a chimney recess): measure the depth and width, and add the rectangle. A niche of 0.90 × 0.25 m adds 0.23 m² of floor.
• A chimney standing inside the room: measure its two sides and the distance from two walls. Subtract the area: a chimney of 0.60 × 0.45 m takes 0.27 m² off the floor. Remember to measure the distance from the same corner as the rest of the room's measurements.
• A bay window: if it's rectangular, it's just one more small rectangle. If it's angled — a trapezoid — measure the short side, the long side and the depth, and the area is (short + long) ÷ 2 × depth. A bay window with 1.80 m outside, 2.60 m inside and 0.70 m deep gives (1.80 + 2.60) ÷ 2 × 0.70 = 2.20 × 0.70 = 1.54 m².

Triangles: the method for truly irregular corners

If a corner is neither 90 degrees nor easy to split into rectangles, any shape can be measured as triangles. Any polygon can be broken down into triangles, and a triangle is uniquely determined by its three sides.

Measure the two walls that meet, and the distance between their far ends — that's the triangle's three sides. Then you can draw it precisely with a pair of compasses, and calculate the area with Heron's formula: halve the perimeter (call it s), and the area is the square root of s × (s−a) × (s−b) × (s−c).

Example: a = 3.00, b = 4.00, c = 5.00. s = 6.00. Area = the square root of 6 × 3 × 2 × 1 = the square root of 36 = 6.00 m².

The three mistakes that wreck an L-shaped room

• Only measuring four figures. An L has six. Without the short leg's two measurements, the room isn't fully defined.
• Forgetting the check measurement. The full external length has to match the sum of the parts — that's the proof the measurements are right.
• Drawing the inward-facing corner as 90 degrees without measuring it. The diagonals settle it. Take them.

And as always: a floor plan can never tell you whether the wall at the inward-facing corner is load-bearing — even though it often is in this kind of room. If it needs to be touched, a qualified surveyor or an engineer needs to look at it on site.