Bathroom renovation in London: when you need drawings

Bathrooms are the most heavily regulated small room in a British house. In the space of four square metres you have electricity next to standing water, a hot water system that can scald, a drainage connection that has to fall at the right gradient, and a ventilation requirement that exists to stop the room rotting. That is why a bathroom project that looks cosmetic on paper so often turns out to be a building control job in practice.

This guide sets out what actually triggers permission, what triggers building regulations, and where drawings earn their keep. It is written from the perspective of a chartered architectural technology practice that produces the drawings these projects are built from.

Do you need planning permission for a bathroom?

Usually not. Work contained entirely within the existing envelope — stripping out, re-tiling, moving a basin, replacing a bath with a walk-in shower — is not development, so planning permission does not arise.

It changes as soon as the outside of the building changes. A new or enlarged window to serve the bathroom, an external soil vent pipe on a principal elevation, a rear or side extension built to house a downstairs shower room: those are all development. So is almost any external alteration to a listed building, and in the conservation areas and Article 4 zones that cover much of inner London, permitted development rights you would otherwise rely on may have been withdrawn. Check the designation before you assume.

Which building regulations apply to a bathroom?

Four parts of the Building Regulations do most of the work in a bathroom, and between them they catch nearly every job that is more than a straight swap.

Part P — electrical safety

All fixed electrical work in a bathroom is notifiable. New circuits, relocated lighting, a shower unit, an extract fan, underfloor heating — all of it. Use an electrician registered with a competent-person scheme such as NICEIC or NAPIT so the work can be self-certified rather than needing a separate inspection.

Part G — hot water and water efficiency

Part G governs unvented hot water storage, and it caps the temperature at a bath tap at 48°C in new dwellings, which in practice means a thermostatic blending valve. It also sets the water-efficiency budget that new-build and change-of-use projects have to meet.

Part F — ventilation

A bathroom needs mechanical extract, whether or not it has an openable window. The intermittent rate is 15 litres per second; continuous systems run lower but never stop. Ducting a fan to an outside wall or through a roof terminal is a detail worth drawing rather than improvising, because the run length and bend count decide whether the fan actually achieves its rated flow.

Part H — drainage and waste

This is the part that most often decides whether a proposed layout is buildable. Waste pipes need a workable fall, a WC needs a short and direct connection to the soil stack, and a new stack or a macerator changes what building control wants to see. Move a WC more than a metre or two and the drainage, not the tiling, becomes the governing constraint.

When do you actually need drawings?

The honest answer is that a same-position refit does not need them, and a layout change does. The moment the WC moves, a stud wall goes in, an en-suite is carved out of a bedroom, or a bathroom appears in a loft or an extension, you are making decisions that are expensive to reverse once the floor is up.

Drawings for a bathroom project typically cover the setting-out plan with fixed dimensions to every sanitaryware centre line, the drainage layout with falls and connection points, the electrical layout with the IP zones marked, the ventilation route, and the tiling and waterproofing extent. That package is what lets a bathroom fitter, an electrician, and a plumber all price the same job and build it without improvising around each other.

If the layout moves, draw it. A relocated soil connection discovered on site is a two-week delay; discovered on a drawing it is a ten-minute conversation.

Who does what on a bathroom renovation?

A bathroom is a small room with an unusually long trade list, which is why sequencing matters more here than almost anywhere else in a house.

Bathroom specialist or fit-out contractor

Bathrooms reward specialists. The work is dense, the tolerances are tight, and the difference between a competent job and a good one is almost entirely in the waterproofing and the tiling setting-out — neither of which is visible once it is finished. A dedicated bathroom renovation firm such as London Renovate handles design, supply, and installation as a single package across London, which removes the usual argument about whether a fault is the fitter's or the supplier's. Whoever you use, book them against a drawing rather than a mood board.

Plumber and heating engineer

First fix — pipe runs, soil connection, valve positions — happens before boarding and tiling. Second fix — taps, shower valve, radiator or towel rail, sanitaryware — happens after. Anything touching gas must be on the Gas Safe Register.

Electrician

First fix cabling before boarding, second fix after tiling. Every fitting has to suit the IP rating for the zone it sits in, and the whole lot is notifiable under Part P.

Tiler and waterproofing

Tanking behind the shower and across the wet-zone floor is the single most important thing that happens in a bathroom, and the only one nobody will ever see. Do not let it be squeezed to save a day.

What order does the work run in?

1. Design and drawings — layout, drainage, zones, ventilation route.
2. Building control notification — full plans or a building notice, before work starts.
3. Order sanitaryware — the longest lead time on most jobs; order before the strip-out, not after.
4. Strip out — and inspect the floor, joists, and existing pipework while everything is open.
5. Structure and studwork — new walls, joist notching where permitted, floor levelling.
6. First fix — plumbing and electrics, plus the fan duct run.
7. Boarding and tanking — cement board or equivalent in wet zones, then waterproofing.
8. Tiling — set out from the drawing so cuts land where they are least visible.
9. Second fix — sanitaryware, brassware, lighting, extract, heating.
10. Sign-off and snagging — building control completion certificate, electrical certificate, and a proper snag list.

What does the design side cost?

Design and drawings are a small fraction of a bathroom budget and the part with the highest leverage on the rest. A measured survey and a building-regulations drawing package for a relocated bathroom layout sits well below the cost of a single mid-range sanitaryware set, and it is what stops the far larger construction budget from being spent twice.

Where the bathroom forms part of a larger project — a loft conversion with an en-suite, a rear extension with a utility and shower room — it should be drawn as part of that package rather than separately, so the drainage strategy is resolved once. Our building regulations drawings service covers exactly that scope.

Frequently asked questions