UK regulations require that electrical and plumbing works in the home are undertaken by qualified professionals.
Drainage.
The waste from the toilet presents the greatest challenge because of the size of pipe required. At around 100mm diameter, the soil pipe can be difficult to relocate and so will often dictate the position of your toilet. However it is not impossible, but cost can be a big consideration.
From behind the toilet the soil pipe can exit straight through the wall, can bend down and exit through the floor or it can bend to either side and exit through an adjacent wall... This means that the toilet can be positioned along any wall where the soil pipe can easily be routed to the main soil vent pipe (SVP).
Naturally the longer the distance the more significant the gradient or fall of the pipe becomes. The waste must flow downhill! (You might be surprised how often this detail is forgotten). The ideal gradient is 20mm fall per 1M run of pipe.
Concealing the pipe can be a problem again because of its size, but it can be routed below furniture and maybe the bath. Avoid bends in the pipe which cannot be easily accessed.
A soil pipe can travel below floor level, but only in line with the floor joists unless there is room to drop below the joists. In a solid floor obviously this changes.
Drainage from the shower, basin and bath involve smaller pipes and there is a bit more scope for position, but be careful with the shower. The trap needs to go below the shower tray, meaning the tray either needs to be raised up on a step, or the trap must be below floor level. There are potential difficulties here to ensure good drainage from the shower as again the pipe needs to flow downhill.
At the time of wrtiing we cannot find any regulations about the fitting of bidet waste pipes, but it is our opinion that a bidet waste should discharge into the SVP, not an open gulley.
Where a toilet is required in a place where there is no access to the main SVP, it is possible to use a macerator/pump system which discharges toilet waste under pressure through a small bore pipe. However it is against regulations to use a macerator as the only toilet in a dwelling.
Hot and Cold supplies.
These services are usually delivered through much smaller pipes and are a lot easier to move and conceal. As the pipes are usually the 15mm diameter, they can be taken below floors easily and chased into walls if required.
Of greater significance is the water pressure. Some bathroom fittings will require a high pressure water supply to work, and if connected to a low pressure system may not work at all, or at best deliver water at a very slow rate. Pressure balance, where the pressure is equal (or near) in hot and cold, is important for some mixer taps and also many shower valves. Taps which work with a single lever action are usually sensitive to pressure issues.
Electrical supply.
Regulations in the UK are very strict in respect of electrical safety. However electrical supplies are essential for lighting and sometimes heaters. Whirlpool baths, sauna or steam facilities and some shower systems require an electrical supply, and an electric shower requires a very heavy duty connection. Shaver points, illuminated mirrors, extractors and under floor heat mats are other items to consider when planning electrical supplies.
Ventilation.
It is a good idea to consider ventilation in the bathroom. It is a legal requirement to provide adequate mechanical ventilation for the bathroom in new build projects, extensions and conversions, and also in existing homes where an extractor has existed in a bathroom it must be replaced by an extractor of at least the same power, during a renovation.
