Putting in a spur socket off a ring main. The single socket spur will be fitted inside a stud wall which will have an access hatch.

This socket will then have a 13 AMP plug and RCD for a shower / sauna the other side of the stud wall.

Space is limited so I intended on putting the back box foamed or use no more nails to stick the back box on to the inside of the plasterboard. Is this okay to do so?

Cannot screw as the screw would only go into the 12.5mm PB before it hit the tiles on the other side.

The other option is to cut the plug off and fit a fused spur but this will invalidate the shower warranty.

Thoughts welcome

Pretty certain the rule is still 1 spur outlet per ring main outlet (the two being directly wired to each other), so that part would be OK.

But, as Tony says, under Part P of the building regs any electrical works in kitchens, bathrooms and other rooms involving the use of water (including saunas) need to be properly signed off.

As you have a thin partition (cables closer than 50mm to the surface and subject to various other conditions) you'd probably need an RCD fitted even if it wasn't a kitchen, bathroom etc; as it does involve one of them you'd certainly need RCD protection. So you may also end up replacing your consumer unit too, unless it's already fitted with RCDs.

But as the regs are frequently changing too, you really need to get professional advice.

Posted By: jfbWhat size is the shower? 13amp seems small.

+1,-- sauna heater too. Personally I'd want it on a separate feed to the CU and it's own breaker.

+1 on the part P sign off. I'm not a sparky so here's some unqualified opinions!:

RCDs are supposed to be tested regularly so need to be located somewhere they are easy to see. You may be able to convert the breaker in your existing consumer unit into an RCD by using a rcbo https://www.tlc-direct.co.uk/Products/BGCRB32.html

There should be an isolator switch nearby the bathroom for anyone working on the appliance to see it is isolated.

Wiring behind thin partitions should be located somewhere obvious, so it doesn't surprise anyone later when they drill holes for a towel rail. Often at ceiling level with isolators etc near to the bathroom door.

bathrooms need special attention to earthing. Rules keep changing on this.

AFAIK any alteration to a circuit needs to be signed off by a part P spark or routed through building control. Adding a spur to a ring is definitely a circuit alteration.

For anything bathroom related you really want to add RCD protection if ts not already there.

The people posting on this thread are trying to help you by pointing out that it is not so much about how to fix it in practice, more about what hoops you have to jump through to get it approved. Unless you are replacing an existing sauna unit, this is an alteration of bathroom electrics with all the regulations that come with it.
I think this thread is full of great advice, and although I can only speak for myself, if you decide to ignore all this and install it without complying to the regs or notifiying BC, I am afraid you are on your own.

First post says youre adding a spur??

Off topic, but your OP gives the impression that on the shower side you're tiling straight onto PB.
Inside a shower cubicle, that's a very unwise thing to do, IMO, even with "so called" water resistant PB. Use a more suitable shower lining there are lots of alternatives and some would give insulation, considering it's a Sauna too.