Start with the waste, not the tiles

Almost every loft bathroom that goes wrong goes wrong because the layout was decided before anyone looked at the drainage. Find your existing soil stack first. If it runs up the back wall through the eaves, a bathroom at the front of the house means a long horizontal run across the loft floor — and every millimetre of that run has to fall in the right direction. That single constraint usually decides where the WC and shower can go.

You have three realistic options. You can connect into the existing stack if it's within a reasonable distance and you can achieve the fall. You can run a new stack down an external wall, which often needs a party wall agreement in a mews terrace and may need planning consent if it faces a street. Or you can use a stub stack inside the loft, finished with an air admittance valve, which avoids having to take a pipe up through the roof. That last option is the neatest for compact mews homes, where the roofline is often protected and visible.

Drainage falls: the numbers that matter

Building Regulations guidance asks for a minimum fall of around 18mm per metre on a 110mm soil pipe, and the same sort of figure on 40mm and 32mm waste pipes. In practice, fit 22mm per metre wherever you can. Pipe that only just meets the minimum will be the pipe that blocks in three years' time, and nobody wants to be rodding a waste run that's buried under a tiled floor.

• WC: keep the run to the stack as short and as straight as possible. Bends slow solids down.
• Basin and shower: 40mm pipe, with a trap that gives a decent water seal and is accessible for cleaning.
• Avoid macerators if you can. They allow small-bore pipe to run almost flat, but they are noisy, need a power supply, and fail at the worst moment. Treat one as a genuine last resort, not a design solution.

Also plan for rodding access. A sealed access cap at the head of the run costs very little at first fix and saves a fortune later.

Water pressure is the loft's quiet problem

Water pressure drops as you go up, and the loft is the worst place in the house for it. If your cold water comes from a tank in the loft, the roof space above it is delivering almost no head at all — a shower up there will be a dribble. You need to check this before you buy anything.

• Static pressure: measure at a ground-floor tap first, then test again upstairs to see the loss.
• What you actually need: aim for at least 1 bar, ideally 1.5 bar, and a flow rate of around 12 litres per minute for a decent shower. Below 10 litres per minute feels disappointing.
• Balanced supplies: thermostatic mixers need hot and cold at similar pressures. An unbalanced supply makes the temperature wander every time someone opens a tap downstairs.

If the pressure isn't there, the usual fixes are a mains-fed unvented cylinder, a pumped supply from the existing tank, or a combi boiler upgrade. All of these are far easier to sort out before the studwork goes in.

Ventilation and moisture under a sloping roof

A bathroom in a loft has more surfaces at low temperature than a bathroom downstairs, which makes it more prone to condensation. Extraction isn't optional. Approved Document F asks for 15 litres per second on an intermittent fan, or 8 litres per second if it runs continuously.

Duct runs matter as much as the fan. Use rigid duct rather than the flexible corrugated stuff, keep the run short, keep bends to a minimum, and terminate properly through a roof tile or gable wall. Duct that passes through a cold roof void needs insulating, or the moisture will condense inside it and drip back down.

Tanking is the other half of the job. Anywhere water will be splashing — a wet room floor, a shower with a low tray, a tiled wall behind a basin — should sit on a proper tanking membrane, not just a coat of sealant. Loft floors are often timber, so a small leak becomes a ceiling stain in a bedroom below.

Making the most of tight mews dimensions

Mews houses are narrow, and the loft is usually the narrowest part of all. A typical usable width after insulation and plasterboard is somewhere between 1.8m and 2.2m, so every fixture has to earn its place.

• Shower: a 900mm x 900mm quadrant or offset tray gives you elbow room without swallowing the floor. A 800mm square is the practical minimum.
• Bath: 1500mm or 1600mm short baths are made for exactly this situation.
• WC: wall-hung with a concealed cistern inside a 200mm stud wall frees up floor and makes cleaning simple.
• Basin: keep projection to 400mm–500mm, and use the eaves for shallow storage rather than trying to fit a vanity unit.
• Doors: a pocket or sliding door saves more usable space than almost any other single change.

Keep all the plumbing on one wet wall. One drainage run, one set of supplies, one place to break into the stack. It looks tidier and it costs less.

Headroom, heights and the details worth getting right

Aim for 2.1m of clear height above the shower head, and 1.8m over the basin. A WC can happily sit under a slope where the ceiling is lower, because you're sitting down — just make sure you're not banging your head on the way out.

A dormer gives you a vertical wall for the shower and a proper window, and it transforms how a loft bathroom feels compared with a rooflight-only scheme. Remember that dormers on a front elevation in a mews terrace often need planning permission, and almost any work near a party wall triggers a Party Wall Award.

Finally, wrap the soil pipe in acoustic insulation before it's boxed in. A WC flushing overhead at 6am is a very different experience when the pipe is silent. Leave isolating valves and access panels where you can actually reach them, and keep pipework inside the warm envelope of the loft rather than in the cold void behind the insulation, where it can freeze. Get those things right at design stage and the tiling becomes the easy part.