Sorry but I was a little confused by your post, you’ve put single side extension or double but having read it several times, I presume you mean single or double storey extension? Apologies if I’m making unwanted comments but as you’ve said the floor plan is a nightmare, I thought I’d raise a few points. I can see why you want the extension downstairs and I agree with making the house suitable for you but I think the new plan compromises the living space for these other rooms that won’t be necessarily used as much as living space. In my opinion, a house with 4 bedrooms, needs more living space - the small area of seating in the kitchen diner would put me off buying & I would explore other possible options for the ground floor. For example could the storage space at the back of the garage & a small portion of the garage be combined and used as utility / dog shower as it has direct access to outside? Do you need a shower room downstairs or is it just a useful addition? Although I think your current upstairs layout is fine, I think an en-suite upstairs would be preferable to a shower room downstairs unless you need the downstairs for washing off after the beach / mucky work, etc. Do you need the extra bedroom downstairs - you’ve mentioned you don’t need more bedrooms? If you need a spare room for visitors, could that double as an office. The boot room seems large for its purpose, especially compared to the living space. Maybe by combining some of these areas you could create a little more living space.
Finally, to answer your kitchen question, I think I’d probably have the taller units in the ‘u’ if space allows for door opening, etc as it will create a more open feel when you enter the room and not see the taller units towards the doors.

thank you

sorry for confusion
my head spinning

yes we have decided on single side extension but wasnt sure if doing right thing and should extend up too

the other option is wrap around at back to open up living

i do need 4th as one of older kids at uni and home holidays

thanks for comments

I guess if you extend up, you could have an extra bedroom & bathroom upstairs - it doesn’t have to be an en-suite - and that would leave you more room downstairs for living space.
A wrap- around extension could also work but that can sometimes leave the middle of the house a bit dark.
Having done a full house renovation inc extension & loft conversion in recent years I know how stressful it can be so please don’t apologise. It will all work out fine in the end but it’s def better to work through all the options now rather than later. Good luck with it!