Hi - I agree with some of the others that you should add a wardrobe (preferably fitted) where the white unit currently is. Ideally you want somewhere for clothes but drawers underneath and probably some overhead storage as well I'd say. You say your son is a real book worm and I noticed a lot of books at the end of his bed - could you put shelves up there where the clock and cool sign is? Either regular shelves or ikea picture ledges look nice with books - that's what we used as per attached photo. Also could you get something like a long ikea Kallax (4x1) to fit under the bed? It might be too high but you can get ones from Argos that are smaller I think and with the baskets that fit in each hole they would be perfect for Lego? I think it's a really lovely room and with a bit of sorting could look great and work well for your son

i meant to have the bed over the existing wall and part of the alcove like this, with the blue bit being built to look like a wall, but with access through a simple door... Depending on type of bed headboard you could have recessed shelves built into the storage above and/or beside the bed to act as bedside table.

I think the key here is to get a bed which maximises mattress size but minimises footprint. You also want to have recessed legs if possible as this makes it easier to get around the bed in a tight space. There is an explanation of this on my website (https://aliceclarestudio.co.uk/project/floating-bed/ ). There is also one which I think fits the bill at Urban Size (https://urbansize.co.uk/products/oak-bed ) though it is only UK standard double which personally I think is too small for a bed which is going to regularly have two people sleeping in it! I prefer to specify a UK king size (150cm) or a European size which is 160cm wide. I have a similar issue in my bedroom which is very long (though not as narrow as yours). Which I solved by putting storage behind the head of the bed - rather unusual but may help you here. My headboard is actually drawers! I think whatever you do will be a bit of a compromise so I suggest trying the bed in different places to see where you think there is enough space to get around it.

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.