How Much Does it Cost to Fit an Understairs Toilet/WC/Cloakroom?

Job 1

This is the barest minimum. Fit a new toilet connected to the plastic
soil stack outside, with a handy cold water supply for the flush and make good. Add a basin with cold water.
2 days work + £250 materials;
Additionals (add to price of above)
Job 2
Make a small
stud wall, fit a hatch from the hall to this newly enclosed space. Plasterboard the underside of the stairs and ceiling and the inside of the hall panelling. Plaster all round inside the new space. Fit skirting boards and architraving.
2 days work + £100 materials;
Job 3
Fit a new
double glazed window (this will need to be done legally);
Job 4
Fit a hanging basin with a mixer tap and hot & cold supplies.
 1 days work + £175;
Job 5
Fit a
downlight and internal pull switch;
Job 6
Fit an
extractor fan;
Job 7
Fit a small radiator with a
2 men 1 day + £100 materials;
Job 8
Box in all the pipework, cables and meters
1 man 1 day + £70 materials;
Job 9
Construct a new
manhole nearby, connect the wastes underground and make good
2 men, 3 days + £500 materials + building regs £200.00;

A Price Guide and Information Sheet on Fitting/Installing an Understairs WC/Toilet

Are you sure there’s enough room, particularly headroom for blokes who won’t be sitting down? Can you fit a basin in and still open the door? Is there a ducting route for a fan, if there’s no window? Are there any meters in the way?
Ok, none of the above are relevant, what state is the existing “cubby hole” in? Do you want a small wall to cut off the very bottom of the stairs and possibly create a new smaller cubby hole behind. (I’m a builder, I know everything there is to know about khasi installation. But if you happen to be a professor of English somewhere, do me a favour, email in and tell me what a “cubby” is please). 
Are there pipes, cables and meters/
fuse boxes cluttering the place up? Decide whether you want everything hidden, or do you want to recreate the Pompidou Centre under the stairs. There’s a case for not only leaving stuff visible but in fact highlighting it, actually making the point that this is nothing more than a necessary and functional little space. Use it and get out!
Will you need to plasterboard the underside of the
stairs and the back of the hall panelling, then possibly plaster the whole area?
Is there a light? Not some rubbish bulb holder nailed above the door but a
downlight set in the ceiling. It has to be flush, or some clot, will simply head butt it! Where will the switch be located?
What about ventilation? If there’s a window, does it need changing? Lots of understairs windows are still single glazed!
If there’s no window, you need a fan. Hopefully this will go straight through the outside wall. How will it be switched? Usually they are designed to operate when the light is switched on and then they switch off automatically about two minutes after the light goes off.
Do you want heat? Do you want another radiator, or possibly a small electric heater?
What about a basin? Legally you have to have one but what about the taps? Do you want a mixer? Do you even need hot water? All anyone does is just get a quick blast of water, the hot never gets to the basin, even if the hot tap
is used.
Where is the outside soil stack? Ideally, it’s plastic and just outside, on the other side of the wall, so both the new lavatory and basin wastes, can be taken to it without any bother at all. Good luck with that one! Probably you will need a new
manhole with an underground connection into the sewer line.
If that’s the case, to remain legal, you must get building regulations approval.
What about sound insulation. For this you need solid stuff. The more solid stuff you have, the less the sound travels through. But what’s the point of loads of sound insulation, if you still use the old thin door?