Small bathrooms are often designed in the wrong order.

It is easy to begin with the bath you like, a vanity seen online or a tile sample, then try to make the room accommodate those choices. In a tight space, the better approach is to map the constraints first.

Four things have an outsized effect on what is practical: the drainage route, the door and circulation space, ventilation, and electrical safety. Resolve those before you fall in love with a particular fitting and the room becomes much easier to plan — and much harder to make expensively wrong.

The WC is usually the least flexible fitting in a bathroom plan because it needs a larger waste connection than a basin or shower.

In an existing bathroom, find out where the soil stack or main foul-water connection is, where the current WC connects to it and how the floor is constructed. England's Approved Document H covers foul-water drainage, including pipe sizes, gradients, access and protection, but the actual route in an older house needs to be established on site.

That matters because a WC that looks better on the opposite wall may be much more difficult to connect than the drawing suggests.

The pipe might need to cross joists, pass through a floor zone with little depth, create an awkward boxing-out or take a route that does not achieve the necessary fall. Cutting or drilling structural members is not a casual DIY decision; where structure is affected, the route needs appropriate professional design.

Showers and basins are generally easier to move, but they are not infinitely flexible either. A low shower tray or level-access shower still needs somewhere for the waste to go. In a suspended timber floor that may be straightforward; in a concrete floor it can be a much bigger intervention.

Before choosing the fixtures, ask your installer or designer to mark the likely waste routes on the plan. If keeping the WC close to the existing soil connection saves substantial work, you can then decide whether the design benefit of moving it is worth the cost and disruption.

The cheapest layout is not always the best one, but hidden drainage work is a poor place to discover the budget after the fittings have been ordered.

A compact bathroom is not just a set of objects that fit inside four walls. It is a room in which doors, drawers, shower screens and people all move.

Start with the entrance door. Mark its full swing on the plan. Then do the same for a shower enclosure, vanity doors, drawers and any tall cupboard.

Now ask what happens when two of those things are open together.

A basin may fit perfectly on paper but leave the bathroom door striking its corner. A deep vanity can turn the route to the bath into a sideways shuffle. A hinged shower screen can need space that was not obvious on the product's closed dimensions.

This is where a smaller fitting can sometimes improve the whole room more than a clever decorative trick.

Which? notes that compact sanitaryware, wall-mounted fittings and shorter baths can all help in small bathrooms. But do not choose a mini basin or short bath from dimensions alone. A basin still needs to be comfortable to use without hitting the wall with your elbows; a bath needs to fit the person who will actually use it.

If the entrance door is the main problem, changing the door arrangement can be worth exploring. A door opening outwards, or a properly designed sliding or pocket door, can release valuable floor space. That change needs to be considered in the context of the adjoining room, fire safety where relevant, the wall construction and access to services; it is not an automatic fix.

A useful test is to tape the key fixture footprints on the floor of the stripped room, or reproduce them at full size in another space. The gap that looked generous on a small drawing can feel very different at human scale.

Bathrooms create a lot of moisture in a small volume. Ventilation therefore belongs in the first layout conversation, not the final electrical one.

In England, Approved Document F provides the Building Regulations guidance for ventilation. The 2026 edition applies to buildings subject to the 2026 standards, while earlier versions may continue to apply under transitional arrangements. Scotland, Wales and Northern Ireland use separate regulatory systems, so the exact requirements need to be checked for the nation and project.

For a renovation, the practical questions are simple:

Where will the extract fan or terminal be?
How will the duct reach outside?
Can the route be short and sensible?
Where will replacement air come from?
Will a new ceiling, cupboard or shower position obstruct the proposed route?

An extractor fitted to an external wall can be relatively straightforward. A bathroom in the middle of a plan may need ducting through a ceiling or roof space. Long, crushed or badly routed flexible ductwork can undermine performance, so the route matters as much as the grille you see.

If you are creating a new en suite where there was not one before, this can materially affect the layout. The neatest place for the shower may conflict with the most practical duct route, or the ceiling void may already be crowded with structure and services.

Ventilation also needs to work as a system. An extract fan cannot move much air if the room is effectively sealed with no route for replacement air. Your designer or installer should consider the whole strategy rather than simply choosing the quietest-looking fan.

Do that before tiling and joinery. Retrofitting a better route later is usually messier.

Water and electricity make bathrooms a special electrical environment.

Electrical Safety First advises that bathroom electrical work is subject to particular requirements, including zones around baths and showers, appropriate equipment, RCD protection and restrictions on socket positions. It recommends using a registered electrician for bathroom electrical installation work.

For a homeowner, the important point is not to memorise the zone diagram. It is to involve the electrician while the room can still change.

The position of a mirror with integrated lighting, an illuminated cabinet, shaver supply, heated towel rail, underfloor heating control, fan or electric shower can all affect the electrical plan. Products also need to be suitable for the location in which they are installed.

If you choose a mirrored cabinet after the first-fix wiring has been completed, the cable may be in the wrong place. If you move the shower during the fit, a light that was acceptable in one position may no longer be appropriate in another.

Electric showers are particularly important to resolve early because they normally require a dedicated electrical supply sized for the appliance and installation. That is qualified electrical work, not a DIY add-on.

Ask the electrician to mark the relevant zones and proposed points on the bathroom drawing before walls are closed and tiles are ordered.

Once those four constraints are drawn, the aesthetic decisions become easier.

You know which wall makes sense for the WC. You know how much usable circulation remains. You know where extraction can go and where electrical fittings can safely be positioned. Now you can compare a bath with a larger shower, a 600mm vanity with a narrower one, wall-hung versus floor-standing storage, or a hinged screen with a fixed panel.

At that stage, the product dimensions are being tested against a workable room rather than forcing the room to adapt around a purchase.

For an early planning meeting, take this sequence:

1. Measure the room accurately, including windows, door, ceiling changes and boxed-in services.
2. Identify the existing soil stack and likely waste routes.
3. Draw the door swing and realistic circulation.
4. Establish the ventilation route and regulatory requirements for your nation.
5. Get an electrician's input on zones, circuits and proposed electrical fittings.
6. Position the WC, shower or bath, basin and storage.
7. Check access with doors, drawers and screens open.
8. Only then finalise the models, finishes and tiles.

A small bathroom rarely has enough spare space to hide a poor early decision. That is exactly why the most useful planning work happens before the attractive part begins.