How easy is it to install a loo?

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Not ourselves, we're not that intrepid! if we end up buying a house we're interested in, we're thinking it would be far more practical if there were a toilet on the middle floor, and there might just be room to make a tiny room with the loo in it, cut out of the bedroom on that floor and opening onto the landing. There is a sink in that room currently, but obviously toilets require different pipes! There's also a bathroom directly above, so maybe that helps in terms of plumbing in pipes?

How easy/expensive is it to do this? Is it really disruptive?

Toilets only need a cold water feed (junction from cold water tap pipe) and a waste pipe. If there is an existing waste pipe from the toilet above that could be tapped into you will have no problems.
Cutting out for the door would depend on if that wall is load bareing or not. And you would need a stud wall built and plastered, then the whole area decorated.
Have you experience of living across three floors? A friend of mine found it very impractical with her 3ds's.

Oh, that sounds hopeful! Thanks.

We haven't lived on three floors before and it's the main thing holding us back on buying this house, tbh. But I know lots of people with small children who say it works for them.