I've been thinking about options to cover all basis and came up with this. To allow guests to use a bath/shower the ensuite would be shared. I added a wall in the master bedroom to divide it and add privacy so guests don't have to walk into the bedroom to get to the bath/shower. The upsides are that the ensuite is larger, includes a bath and shower and has more vanity room. Also, by diving the bedroom I can add sort of a walk-in wardrobe. The downsides are that there's only one bath/shower that has to be shared and guests have to walk through the walk-in wardrobe to get to it. Also, it reduces the size of the master bedroom but it's quite a long room anyway and it would cut out the light from the windows in the bedroom. Personally I don't see these as big problems but I guess some people won't like it. Do you think this is a good idea?

If you shop for a space saving toilet there should be plenty of room for a full sized basin on the same wall. A p shaped bath would also give you a little more room at the shower end.

Both Hansgrohe and (Friedrich) Grohe are German companies; Friedrich was a son of Hans who went on his own in the 1930s, so they share a certain heritage. These days, Grohe is larger and no longer family owned. Both produce outside Germany as well as on their original German sites still. Both produce "basic" and "high-end" ranges. They're both regarded well back home in Germany, much of the choice is down to personal preference. Hansgrohe is on average slightly more expensive (but not on everything). They also own the Axor design brand which has a more "ecletic" selection. One can mix-and-match; in our bath (bit messy, sorry - we're using it for the washing up while waiting for the kitchen to be installed), the mixer is Grohe as is the shower wall outlet, the shower rail and head shower arm (just above the shower screen) are Hansgrohe pieces. At least the chromed finishes are very close for the two: Both can be sourced well below retail list prices - if you're prepared to wait, German online sellers for bathroom goods tend to have many-months delivery timelines. For Grohe essentials (the classical Grotherm mixers) at least, any German DIY shop stocks these at very attractive prices (the difference to the UK retail price list will pay for the ferry ticket and fuel).

If you intend staying here for a decade then you should make this house work for you and not the next people. I think that since you have already flagged extending in 10 years you should also future proof your plans now. If it were me I would have a bedroom where the current big bathroom is in order to give you the possibility of later extending over the ground floor extension. I would also want a super sized bathroom for my use - I have always believe the person paying the mortgage should have a better bathroom than people who visit three times a year. And I would take the view that when a loft bedroom is later added it won’t matter if one of the four bedrooms is compact. So I offer an alternative.