the room is currently 2500 * 2900 and would be split into...

800 * 2900
1600 *2900

I've decided that 800mm would be too narrow for my ensuite and have plumped
for 900mm. Luckily my hallway is exactly 900mm wide, so I was able to mock
it up when the suite arrived. 900mm is reasonably comfortable, given that
the basin will be opposite the door, giving extra room for the posterior.
However, 800mm would probably be too narrow. Although 800mm is doable for
the toilet, you'd need to find an extra shallow hand basin.

If you went for 900mm, I reckon you could make a bathroom out of the
remaining 1500x2900, especially as the master bedroom has an en-suite.

It might be possible to shorten the en-suite by a small amount. I can't
remember how long toilets are to check, though. It might not fit. This could
provide a valuable cupboard either for a hot water cylinder (if needed) or
for towel/toiletries storage for the main bathroom.

800 600 900 600

+-------+ +-------+----+
| | |
|TOILET SHOWER |C/B?|
| | | 900
| BASIN | |
+---------+----+-------+ + 100
| TOILET |
| | 800
| |
| +-----------------+
| | |
| | BATH | 700
| | |
+-+ +-+-----------------+
1200 1700

Christian.