Minimum allowance for BTW toilet system

A concealed cistern has to hold the regulation 6 litres of water, so the thinner it is, the wider or higher it will be. I had to change one for a customer recently, they bought one on line for a standard 850 high unit, but it was far too 'high' so chose the correct cistern before building the enclosure. As a soil pipe is only 110mm, this should fit your boxing anyway, personally I would visit your local plumbers merchants and have a look at the cisterns and ask for a little advice. Remember also to give yourself enough 'legroom ' at the side of the toilet, easy to put it too close to a wall making it more difficult to use.

If you have a bend or flexi on the soil then I would allow 200mm regardless of the cistern depth, with the rubber and collars they are more than 110mm and it’s tricky if space is tight. If the soil can go straight out through the wall then you could go for the bare minimum depth of the cistern, but then just be aware that the cistern flush pipe sometimes have a sweeping bend which sometimes will need to extend further forward than the front of the cistern. Build a Support for the back of the flush pipe so that it can’t move back, the bungs don’t always stay in place. Use a cistern with an accessible flush plate or make provision for repairs if it’s a small flush. You could have a removable shelf above for instance, but if you are building the cistern into a full height wall then definitely go with an accessible flush plate.