Is the area opposite the understairs cupboard a storage cupboard? I would look at placing the toilet in there if possible. I'm not keen on the restricted head height under the stairs for a cloackroom.
We moved the electric board over a little so it's under the lower part of stairs, then added WC under the stairs. Plenty of headroom when you walk in, less above toilet where you sit down - it's totally fine. But it depends on your overall ceiling height and the design of your stairs. That cupboard opposite looks too small, but worth checking measurements. Also worth checking where existing soil pipe is and access requirements to get a new WC hooked up for water in and soil out.
Isla Cherry
CWD
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