All articles · 5 min read

How do you draw a first floor with sloped ceilings on a floor plan?

Draw the knee wall as a real wall, write two heights on every room, and add a dashed line where the ceiling is high enough to stand up straight. Then the drawing stops promising more than the floor can deliver.

Why an ordinary floor plan isn't enough on the first floor

A floor plan is a horizontal slice, drawn as if you were looking straight down at the floor. It shows where the walls sit and how big the floor is — but a floor has no height on paper. On a ground floor that doesn't matter, because the height is the same everywhere. On a first floor with sloped ceilings, the height is the most important thing about the room, and that's exactly what the drawing can't show.

The result is a drawing where 48 m² of attic floor looks like 48 m² of living room. The two aren't comparable: in the attic you might only be able to stand up straight on 30 of those 48 m². So a floor plan of a sloped-ceiling first floor needs to be supplemented — otherwise it lies, without anyone meaning to mislead.

The three things the drawing needs

1. Draw the knee wall as a wall

The knee wall is a real, vertical wall with a real thickness and a real position — typically 90-120 cm high and set 1.0-2.0 m in from the facade. Draw it as a wall, not as a thin guide line along the edge. It's the single most important line on the whole floor: it separates room from knee-wall space, and it's the surface built-in wardrobes and knee-wall hatches get measured against.

If there's no knee wall at all — the sloped ceiling runs all the way down to the floor — write that down. The absence is just as important a fact as the presence.

2. Two heights per room

Write in every room:

• The height at the knee wall, e.g. "knee wall 0.95 m"
• The height under the ridge or flat ceiling, e.g. "ceiling 2.30 m"

If the two sides of the room differ — and in an older house they often do — write down both numbers. Two extra figures on a drawing cost nothing and remove most of the doubt.

3. The height line, dashed

Draw a dashed line where the ceiling reaches 2.0 m of clear height. That's the line for where an adult can stand upright, and it's the one people feel when they live there: everything outside the line is room for a bed, a chest of drawers and wardrobes — not for standing up in.

You find the line like this: distance in from the knee wall = (desired clear height − knee wall height) ÷ the ceiling's rise per metre. With a 0.90 m knee wall and a 45° roof, where the ceiling rises 1.00 m per metre, the 2.0 m line sits (2.00 − 0.90) ÷ 1.00 = 1.10 m into the room, on each side.

The same formula gives you any other height line you might need. Officially usable floor area under a sloped ceiling is counted only from a certain clear height — and which height that is depends on where the home is; in the UK, building regulations commonly use 1.5 m as the point of reference for a habitable room. Put that figure into the formula if you want that line on the drawing too: (1.50 − 0.90) ÷ 1.00 = 0.60 m into the room.

Write the area as two figures, not one

A room with a sloped ceiling should carry two areas on the drawing:

• Floor area: the whole floor, wall to wall. E.g. 6.00 × 8.00 = 48.00 m².
• Area with full standing height: the area inside the height line, e.g. 3.80 × 8.00 = 30.40 m².

Two figures instead of one clears up the most common confusion on an attic floor — that the official floor area and the sum of the rooms "don't match". They match fine; they just measure different things. The register's figure is even measured outside the external walls, while you measure internally.

Dormers, the stairwell and the chimney

• Dormers should be drawn as the niche they are, with internal width and depth. A dormer lifts the ceiling locally, so the height line jumps outward right there — draw the jump, or the dormer will look like a window with no effect.
• The stairwell is drawn as a hole with its own dimensions, ideally with an arrow for the direction of travel. It typically sits in the tall zone and eats up 4-5 m² you would otherwise have counted as usable.
• Chimneys and duct boxes should be drawn in too, even when they stand inside the knee-wall space — they divide it into sections and set how deep any built-in storage can go.
• Roof windows are marked in the sloped ceiling. They let in light but don't move the height line a single centimetre, and that's worth being able to see on the drawing.

Doing it in practice

If you scan the first floor with your iPhone's LiDAR in HouseSense, you get the room's internal measurements and the walls' positions — and the knee wall comes along as the wall it is, standing as a real, vertical wall in the room. If you'd rather draw the floor yourself, you can draw it on the digital graph paper with your finger, put the knee wall exactly where it stands, and have the areas calculated from the internal measurements.

You can read off the heights by holding your finger down on a wall: the wall folds open and is shown from the front as an elevation, with its measurements and openings. That's the view that turns the knee wall's height and the roof window's position into something you can write on the drawing instead of guessing.

The height lines, on the other hand, are something you add on top of the measurements yourself — work them out with the formula above, and write them on. And if the drawing is going into a building application, an official registration of the home, or a sale, it's your preparation, not an approval: the authorities decide the registration, and a qualified surveyor or engineer decides what the structure allows. The drawing may be dressed up; the figures may not.