Bathroom electrical zones, explained properly

Section 701 of BS 7671 carves a bath or shower room into zones based on one simple question: how likely is the kit in that spot to get wet? The closer to the water, the harder the rules bite. Nail the zones and everything else, IP ratings, which fittings you can use, where the sockets go, drops into place.

Zone 0: in the water

The inside of the bath or the shower tray itself. Anything here can end up submerged, so the minimum is IPX7 and the only equipment allowed is SELV at a maximum of 12 V AC or 30 V ripple-free DC, with the safety source sat outside the zones entirely. In practice that means almost nothing goes here beyond purpose-made bath lights and whirlpool kit.

Zone 1: above the bath or tray

From the rim of the bath, or the floor of the shower tray, up to 2.25 m. Minimum IPX4, rising to IPX5 where water jets are used for cleaning. Shower units live here by necessity, and fans and luminaires are fine if they're rated for zone 1. What you can't have is any standard 230 V accessory, and definitely no socket-outlets. If you're not sure what a rating on a box actually means, run it through my IP rating checker.

No tray? Wet rooms move the lines

Rip the tray out for a level-access wet room and the textbook diagram changes. With no basin to measure from, zone 1 becomes a 1.2 m horizontal radius around the fixed water outlet, still up to 2.25 m above the floor, and there is no zone 2 at all, the bigger zone 1 swallows it. If the space gets hosed down for cleaning, the minimum steps up to IPX5. Wet rooms are exactly where "measure the actual room" stops being advice and becomes the job.

Zone 2 and the socket question

Zone 2 stretches 0.6 m horizontally beyond zone 1, again up to 2.25 m. Minimum IPX4. The one accessory with a special pass is the shaver supply unit made to BS EN 61558-2-5, that's why it's the only "socket" you'll ever see next to a bathroom mirror. Standard 13 A socket-outlets must be at least 2.5 m horizontally from the zone 1 boundary, Regulation 701.512.3. You'll still hear this quoted as "the 3 m rule", that was the 17th Edition figure and it refuses to die, the 18th Edition trimmed it to 2.5 m. Either way, measure it out in a typical UK bathroom and you'll see why the honest answer to "can I have a socket in here?" is nearly always no.

RCDs and bonding

Under the 18th Edition, every circuit of a location containing a bath or shower needs 30 mA RCD additional protection. Not just the shower circuit, all of them, lighting included. Supplementary bonding can be omitted where the RCD and main bonding conditions are met, but that's an engineer's judgement, verify it on site rather than assuming. Miss it and it's exactly the sort of thing that gets coded on an inspection, see my guide to EICR codes explained for how hard.

Part P: the paperwork side

Zones decide what you can fit. Part P of the Building Regulations decides what you have to report. In England, a room containing a bath or shower is a special location, so a new circuit, a consumer unit change, or an addition or alteration within the zones is notifiable, either self-certified through a competent person scheme or run past building control before you start. Like-for-like replacements are maintenance and generally aren't. Wales has its own version of Part P and Scotland uses a different building standards system entirely, so check the rules for where the job actually is, not where you trained.

The bits that catch people out

• A fixed shower head over a bath changes the zone extents, don't assume the textbook diagram matches the room in front of you.
• A ceiling above 2.25 m technically puts fittings outside zones 1 and 2, but a steamy bathroom is still a steamy bathroom, decent IP is still the smart call. Planning the lighting layout? My downlight spacing calculator does the sums.
• Towel rails should be fed via suitably rated fused connection units sited outside the zones, not plugged into some bodged arrangement behind a panel.