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Can you fit a bathroom under a sloped ceiling?

Yes — but the shower decides everything. It needs around 2.10 m of headroom, while the toilet and basin can move further out under the slope. Here are the heights that decide what can go where.

The height per fixture — the figures that decide the layout

A bathroom under a sloped ceiling is not planned by square metres, but by where the body has to stand, and where it has to sit or lie. Work with these clear heights above the finished floor:

• Shower recess: about 2.10 m. A fixed shower head typically sits 2.00–2.10 m up, and you need to be able to stand upright underneath it. It is the bathroom's tallest requirement.
• Basin: about 1.90–2.00 m where you stand in front of it. The basin itself sits at 0.85–0.90 m, but it is your head that decides.
• Mirror: 1.50–1.80 m from the floor to the top edge. A mirror on a sloped surface tilts and shows the ceiling — mount it on a vertical surface instead.
• Toilet: about 1.30 m above the front edge of the seat, where the head is when seated. The seat itself sits at about 0.42 m. Allow 1.80–1.90 m where you stand up — that is, 40–50 cm in front of the bowl.
• Bath: about 0.55 m at the rim, but 1.90 m where you stand when stepping in and out.
• Washing machine, front-loading: about 0.85 m to the top, but you need to be able to bend down in front of it — allow 1.50 m over the standing area.

The calculation: how far in does each fixture need to stand?

Take a first-floor bathroom with a 90 cm knee wall and a 45° roof pitch. The ceiling climbs 1.00 m for every metre you go in from the knee wall, so the distance in from the knee wall is:

distance in = required height − 0.90 m

• Shower (2.10 m): 1.20 m in
• Basin, standing area (1.95 m): 1.05 m in
• Toilet, where you stand up (1.85 m): 0.95 m in — but the bowl itself can sit 0.40 m in, since only 1.30 m is needed over the front of the seat
• Bath, rim (0.55 m): 0 m in — the bath can sit right up against the knee wall, as long as the standing area beside it has 1.90 m

Put that on an actual room: the bathroom is 2.40 m wide and 2.20 m deep, with the knee wall along one long side and a gable wall at one end. The shower needs to stand at least 1.20 m in — that is, in the half of the room furthest from the knee wall. With only 2.20 m of depth in total, the shower eats up almost the entire tall zone.

The conclusion is nearly always the same: the shower goes against the gable or under the ridge, everything else moves out under the slope. A bath under the sloped ceiling is actually the most space-efficient solution in an attic room, because you lie down exactly where it is low.

The mistakes that cost the most

• Placing the shower by the drain instead of by the height. Drains can be moved; the ceiling cannot. Lay out the plan by the heights, and let the pipework follow.
• A door that swings into the sloped ceiling. An ordinary door is a good two metres tall, cannot sit in a sloped wall, and can barely swing towards it either. Put the door in a vertical wall, and let it swing into the tall zone — or use a sliding door.
• The basin mounted against the knee wall. The basin fits fine at that height, but you end up with your head in the sloped ceiling every time you lean towards the mirror.
• No room to open the cabinet door. A 2.00 m tall cabinet needs to stand 1.10 m in, and will often block the standing area at the basin.

Moisture and ventilation: the part you should not decide yourself

A bathroom in an attic room is a wet room built into a structure that already has to manage moisture from the house and ventilation of the roof. Three things belong to a professional — every time:

• The vapour barrier. The sloped ceiling usually has a vapour barrier behind the boards, and it has to stay unbroken. Every screw, pipe penetration and built-in recess is a potential hole, and moisture from a bathroom getting into the roof structure is expensive to discover too late.
• Ventilation. A bathroom needs mechanical extraction, and in an attic room the outlet has to be led all the way outside — not just up into the roof space. At the same time, the roof's own ventilation through the knee-wall space must not be blocked.
• Waterproofing and falls towards the drain. Wet rooms are regulated work in most places, with requirements for membrane and workmanship. It has to be carried out and documented by a professional with the right qualifications.

On top of that, a new first-floor bathroom usually needs approval from the authorities before work starts, and changes to a home's floor area and fixed installations have to be recorded in the public building register. And whether a wall can be removed to make room — that is decided by a building surveyor or engineer, never by a drawing.

Does the bathroom count towards the home's floor area?

The floor in a bathroom under a sloped ceiling is counted the same way as the rest of the first floor: official figures only count floor where there is a certain clear height to the ceiling. Where that line falls is set by the authorities where the home is located — but the calculation is the same everywhere. Set the line at 1.5 m, for example: a bathroom of 2.40 × 2.20 m with a 0.90 m knee wall and a 45° roof then has 0.60 m along the knee wall below the line — that is 2.40 × 0.60 = 1.44 m² that does not count, even though the bath stands right there.

That does not change whether the space is usable. It only changes the figure recorded in the public building register — and it is the authorities who keep that record.

The measurements to bring to the plumber

1. The room's internal floor measurements, length and width.
2. The knee wall's height, measured from the finished floor at both ends.
3. The height at the ridge or flat ceiling.
4. The horizontal distance from the knee wall in to full height.
5. The location of the existing soil stack and drains — that is the most expensive thing to move.

Once you have scanned the room with your iPhone's LiDAR in HouseSense — or drawn it on graph paper — you have the internal floor measurements and the walls' positions, and you can try out the layout on the floor plan, where everything is drawn to scale. Hold your finger down on a wall, and it folds up and shows itself from the front with measurements and openings, so the knee wall's height and the windows' positions can be read off instead of remembered. And in the 3D model you can walk into the room at 170 cm eye height and see exactly where you have to duck — before the shower ends up in the wrong place.