Your Architect’s Plans Are Done. Who Designs the Bathroom?

By Jonathon Barclay, Founder Published: August 2026 12 min read

Ensuite bathroom designed after an architect's plans: terrazzo floor, oak vanity, zellige shower, brass fittings, LED mirror. — Tissino’s showroom ensuite. Every fixture position was resolved before the tiler started.

Your architect’s drawings show the room and the walls, and a building-regulations set normally shows the drain connection too. Tap centres, waste falls, fixing heights and tile set-out usually sit in a separate fit-out package, and that’s a bathroom designer’s work, normally through a showroom, once the plans are signed off. A separate fitter or contractor then builds it.

The drawings show a rectangle, a door, a window and a soil pipe. Now look for where the shower valve sits, or a line for the fall of the floor to the drain, or a grid for the tiles behind the bath. On a planning and building-regulations set, that detail usually isn’t there. The gap is normal, and it isn’t a failing on the architect’s part: it’s simply outside the scope of a typical architect appointment. The person who fills it is a bathroom designer, working through a showroom.

Your extension is approved, the drawings are signed off, the builder has a start date, and first fix is a few weeks away. Somewhere in that handover, the bathroom quietly stopped being anyone’s job. The architect drew the room and the builder is waiting to be told what goes in it, so you assumed one of them owned the detail in between. Often neither of them does.

This guide names every link in the chain: architect, bathroom designer, fitter or contractor, building control. It sets out what each one is responsible for and what each one is regulated by. Then it gets practical, covering when to bring a bathroom designer in, what has to be locked before first fix, and what to send a showroom before you visit. It finishes with the question that always follows the first one: who fits it.

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What’s Actually Missing From an Architect’s Bathroom Drawings?

What your architect draws depends on what you appointed them to do. A planning and building-regulations set is built to get permission and satisfy building control, so it shows the room, the walls and the window and door openings. It normally shows how the waste connects back to the stack too. That drawing won’t tell a tiler where to start a course, and it won’t tell a plumber where to set a tap. That level of detail sits in a fit-out package, and it’s a separate appointment: RIBA and the BIID publish a separate contract for interior design services. That tells you it’s distinct work. If you want it included, ask for it in writing before the appointment is signed.

A planning or building-regs drawing typically stops short of tap centres, fixing heights, shower falls, tile setting-out and niche positions. Some architects will cover all of it, so ask.

In practice, a handful of specific things go undrawn: the height and centre line of the shower valve, the exact waste position under the tray, and the fall of the floor towards it. Then there’s the noggin behind a wall-hung basin, and the height it sits at. Where the tile grid starts matters too, so the cuts land somewhere sensible, and so does whether the niche fits between the studs or needs them moved. The drawing we’re handed most often shows the room and the drain, and nothing past that.

That’s also why the useful phrase here is “bathroom designer” rather than “interior architect”. Architect is a protected title in the UK, so the word buried inside “interior architect” isn’t open to anyone to use. Bathroom designer is a plain description of a role, and it’s the role this handover needs.

Every decision that sits in that gap is worth writing down before you start, and our planning checklist sets them out in order.

Appleton Brooks brass shower valve mounted precisely on a square zellige tile grid — A shower valve landing dead-centre on the tile grid is not luck. It is a dimension someone set before the tiles went up.

Who Designs What: Architect, Interior Designer, Bathroom Designer

Three different people can hold a pen over your bathroom, and only one of them is normally paid to set out the services. The titles overlap in conversation, but they don’t overlap in scope.

	Architect	Interior designer	Bathroom designer
Layout	Room size, wall positions, window and door openings	Feel, finishes and furniture within the agreed shell	Fixture positions inside the room, to the millimetre
Services set-out	Waste connection back to the stack	Not usually included	Tap and valve centres, waste positions, falls, niches, duct routes
Product specification	Only if you appoint them for it	Often, on finishes and furniture	Every item, by model and code
Supply	No	Sometimes, through their own accounts	Usually, through a showroom
Regulated by	ARB, under the Architects Act 1997	Nothing statutory; BIID is voluntary	Nothing statutory; the title is a role, not a qualification

“Architect” is a protected title in the UK. Under section 20 of the Architects Act 1997, you can’t practise or trade under a name containing the word unless you’re on the Architects Register. The Architects Registration Board is clear that the work itself isn’t restricted, so anyone, architect or not, can legally provide architectural services in the UK. The Act allows only three exceptions: naval architect, landscape architect and golf-course architect. “Interior architect” isn’t one of them. The British Institute of Interior Design states it plainly: it’s illegal to call yourself an interior architect in the UK unless you’re ARB-registered.

Interior design itself isn’t statutorily regulated, so anyone can call themselves an interior designer, whatever their training. The BIID is a voluntary professional body, and it runs its own registered mark for members who meet its standards.

One row is missing from that table on purpose. Building Regulations compliance isn’t automatically anyone’s job by title, and Approved Document P puts it plainly: the people responsible for building work must make sure it complies. That means, for example, the agent, the designer, the builder or the installer, and the building owner may be responsible too. If the work does not comply, the building owner may be served with an enforcement notice.

For the full breakdown of designer versus fitter, and where one stops and the other starts, read what a bathroom designer does day to day.

When Should a Bathroom Designer Come In?

As soon as the architect’s drawings are signed off, and before the builder starts first fix. When drawings already exist, two things change: the measure-up is short, because the dimensions are already on paper, and the deadline isn’t yours, because the order date is driven by the builder’s programme. So the first thing we ask for is the drawing set rather than a site visit. That’s where a design service that works off drawings begins.

Drawings In

You send the architect’s plan and elevations to the showroom. A PDF works, though a CAD or DWG file is better. Check the revision number first, because working from a superseded drawing means the design has to be done again.

Design Developed Against Them

The bathroom designer sets the fixtures inside the shell the architect drew. Because the dimensions are already on paper, the measure-up is short, and sometimes we skip it altogether. Nothing moves on until you’ve signed off the layout.

Set-Out Pack Issued to the Fitter

The designer produces dimensioned plans and elevations: centre lines, heights, falls, niche positions and the tile grid. The pack goes to a contractor who’s already appointed, so it has to reach them before first fix rather than on the day.

Order Placed

Products are ordered against the signed-off design, and long-lead items set the date: stone, made-to-order furniture, certain brassware finishes. On an architect-led job the builder’s programme sets that date, and it isn’t yours to move.

First Fix

The contractor builds to the pack. Pipes, wastes, cables, noggins and the wall build-up all go in at positions someone has already drawn. Anything still undecided by now gets decided on site, by whoever’s holding the tools.

This section only covers what changes when an architect is already involved, and the wider sequence, from first appointment through to handover, is in our full renovation timeline.

What Has to Be Locked Before First Fix, and What Can Wait

Everything that gets buried has to be decided first. There’s no single legal definition of first fix and second fix, but across the industry the split is consistent enough to rely on. First fix is everything that gets buried: hot and cold pipe runs, waste runs and their falls, cable runs and back boxes, noggins and fixings, and the wall build-up behind the tiles. Second fix is everything you see: sanitaryware, brassware, shower valves, accessories, switches and sockets. NHBC’s Pride in the Job best-practice guide makes the point that the spacing of electric sockets and switches is fixed at first fix.

The list on the left isn’t about taste. Every item on it changes how a wall or a floor gets built.

Lock before first fix

Tap and valve centres, and their heights
Shower waste position, and the fall of the floor to it
Niche and recess positions
The tile setting-out grid
The extractor duct route
Underfloor heating manifold position, if you are having it

Can wait until second fix

Tile or stone finish, within the agreed format size
Brassware finish
Mirror and cabinet model
Light fittings
Accessories

Second-fix choices don’t change how a wall is built. First-fix ones do.

Calypso wall-mounted brass tap set into marble tile above a wall-hung vanity basin — The tap position, the pipe route and the tile setting-out all had to agree before this wall was tiled.

What to Send the Showroom Before Your First Appointment

Send everything the architect generated, because that’s the part a showroom can’t recreate from a site visit. A designer can measure a room, but a designer can’t re-derive your approved drawings, your engineer’s calculations or your window schedule.

The architect's PDF plan and elevations, at the current revision
The CAD or DWG file, if you have it
The scale stated on the drawing
The planning decision notice and the approved drawings
Any structural engineer's calculations
The window and door schedule
Joist direction and span, if it is a timber floor
Ceiling heights
Where the soil stack and the water supply come into the room

The item we chase most often is the structural engineer’s calculations, if the extension needed them.

One thing worth checking before you send anything: drawings get revised, and the revision number matters more than the date on the file.

That list is the architect-specific part. Read what happens at a design appointment for everything else worth taking with you, and for what happens once you’re there.

What Actually Goes Wrong When Nobody Owns the Set-Out

The failure is rarely a bad decision. It’s an undocumented one, and we see it most on jobs where nobody was asked to send a drawing. When the set-out is never formally instructed, positions still get chosen quickly, verbally, by whoever’s on site when the question comes up. Nobody’s acting in bad faith. There’s simply no drawing to check the answer against, and no record of who agreed it.

The specific layout errors that follow are worth knowing on their own, and we cover the ten most common bathroom layout mistakes in a separate guide.

A set-out drawing puts every centre line, height and fall on paper before anything is buried. It gives the fitter one version of the truth to work to, and it gives you the chance to change your mind on paper rather than in plaster. Changes are cheap on a drawing. They’re expensive once the walls are closed.

When nobody is instructed to produce the set-out, the set-out still happens. Whoever calls a fixture position out loud on site, on the day, becomes the designer of that detail, and there’s nothing written down to check it against afterwards.

That matters most at handover. If a position is wrong and no drawing exists, the conversation has nowhere to go, because everyone remembers the day slightly differently.

So Who Actually Fits the Bathroom?

Usually not the people who designed it. Design and fitting are normally two separate firms: a showroom designer specifies the products and produces the set-out, and a separate fitter or main contractor builds it. That’s a common arrangement rather than a compromise, but it does mean somebody has to connect the two.

On an architect-led job that connection is easier to get wrong, because the contractor is usually appointed already. You’re not choosing a fitter after the design is finished; the builder is on site, on a programme, with a start date. So the set-out pack has to land with that contractor before the walls go up. A pack that arrives after first fix is a record of what should have happened.

Ask two questions early: who receives the set-out pack, and by what date. Then put the answer in an email.

Bowman Bathrooms designs and supplies on a “your fitter or ours” basis. You can use your own contractor, which is the obvious route when an architect and a builder are already in place, or you can use an installer from the vetted approved-installer network. Either way the set-out pack gets produced and issued. You can read how a Bowman Bathrooms project runs before committing to anything, and the bathroom design service page covers what’s included.

The architect drew the room. Somebody still has to draw what goes inside it, and your builder needs that drawing in hand before first fix.

Bowman Bathrooms designs and specifies bathrooms from architects’ drawings at its showrooms in Braintree and Leigh-on-Sea. Bring the plan and elevations, the planning decision notice, any engineer’s calculations and the window schedule, and we’ll work to the revision you’re building on site. The design work is included in the project, so the only thing you pay for is the products you choose.

Frequently Asked Questions

Common questions about architects, bathroom designers and who does what

They can design the drain connection, and a building-regulations set normally shows how the waste runs back to the stack. The position of the shower waste inside the room sits in fit-out detail, and so does the fall of the floor towards it. That’s a separate appointment unless you asked for it in writing.

Dimensioned plans and elevations. They need to show centre lines, heights and falls: tap and valve centres, the waste position, noggin and bracket heights, niche positions, the tile setting-out grid and the extractor duct route. Without those, positions get chosen on site and there’s nothing to check them against.

Once the architect’s drawings are signed off, and before the builder starts first fix. The design has to be finished early enough for the set-out pack to reach the contractor, and early enough for long-lead products to arrive within the builder’s programme.

The pipes, wastes, cables and noggins still go in, and somebody decides where, usually on site and usually quickly. Moving those positions afterwards means opening up the wall or the floor again. Changes cost nothing on a drawing. Once the walls are closed, they’re expensive to move.

Not usually, and scope varies from practice to practice, so ask directly. Interior design in the UK isn’t statutorily regulated, and anyone can use the title. The BIID is a voluntary professional body with its own registered mark for members who meet its standards. Services set-out is more often a bathroom designer’s work.

The RIBA Plan of Work 2020 splits a project into eight stages, 0 to 7. Stage 3 is Spatial Coordination, Stage 4 is Technical Design and Stage 5 is Manufacturing and Construction, and Stage 4 is where you want bathroom decisions made. The Plan of Work sets out stages rather than fittings. Treat it as a marker, not a rule.

Installing a new circuit is notifiable. So is any addition or alteration to an existing circuit inside the special location around a bath or shower. Approved Document P sets that location out as 2.25 metres up from finished floor level, and 600 millimetres out from the edge of the bath tub or shower tray. Notifiable work can be self-certified by a registered competent person, certified by a registered third party, or notified to a building control body. Approved Document P requires the work to follow BS 7671, the IET Wiring Regulations.

At Bowman Bathrooms the design work is included, so you only pay for the products you choose, and fee models vary widely across the rest of the industry. Our guide to whether a bathroom designer is worth it explains how the different arrangements work in practice.

Still got a question the FAQs didn’t answer? Book a showroom visit below and a designer will walk you through it.

Miller from Sweden finished bathroom with stone-effect tile, wall-hung vanity and mirror cabinet — The payoff of getting the coordination right: nothing crowded, nothing off-grid.

Turning the Drawings Into a Bathroom

Your architect drew the room. A bathroom designer still has to draw the fixtures inside it, and that drawing has to reach your builder before first fix. Bowmans designs and specifies bathrooms from architects’ drawings, working to the revision you’re building on site. The design work is included, so you only pay for the products you choose.

Book a bathroom design appointment Or book a showroom visit →

Your Architect’s Plans Are Done. Who Designs the Bathroom?

What’s Actually Missing From an Architect’s Bathroom Drawings?

Who Designs What: Architect, Interior Designer, Bathroom Designer