another option would be a single garage with a playroom and utility at the back and keep you current downstairs lay out
Personally I feel utility rooms are over valued and you need a really big house for it not to be better used.
I had a 3x3m utility at the back of the house that I sacrificed for larger kitchen/breakfast/play area and bigger windows.
This is my utility now, cupboards keep things fairly quiet and they are full sized appliances
Not sure if already address but why are the roofs not the same height?
add a small single storey extension to the side of the existing kitchen
Those cupboards for full size units are a good idea. I assume the worktop etc needs to be deeper?
Was it off the shelf cabinets etc?
I am now considering doing the extension in two stages. Stage 1 add a small single storey extension to the side of the existing kitchen, this is the most practical position for this room anyway. Then Stage 2 add the two storey extension to increase the size of the kids bedrooms upstairs, they are still young so no massive hurry with this. The addition of a utility room now would be far more useful.
This way I get to keep my current kitchen so no need to budget in installing a new one. When I get to phase 2 I would consider knocking through and creating a kitchen diner.
I am now considering doing the extension in two stages. Stage 1 add a small single storey extension to the side of the existing kitchen, this is the most practical position for this room anyway. Then Stage 2 add the two storey extension to increase the size of the kids bedrooms upstairs, they are still young so no massive hurry with this. The addition of a utility room now would be far more useful.
Not sure if already address but why are the roofs not the same height?
From what I have read it is because
"Planners prefer house extensions that are visible from the road to be ‘subsidiary’ to the existing house. In effect, the ridge height of the new part of the building is expected to be lower than the main ridge height."
Whether this is true or not I do not know.