Looking at it, I think there are several possible options but I think you’d have to lose one of the en-suites from either bedroom 1 or 2. You could create a corridor through either of those bedrooms to the new master suite but you would of course cut off access to one or other of the en-suites. It could remain as a bathroom but it would be across the corridor. However, that creates a long, potentially dark corridor. So, another option would be (if possible) to remove the en-suite next to the landing (bedroom 2 en-suite I think) & open that up to the landing area. This could be home to a desk / small office area if needed. Then put a corridor through the en-suite for bedroom 1. The en-suite would need to be smaller & need reconfiguring but would probably still work - probably remove the bath & put in a shower instead.
If you really didn’t want to remove the en-suites, I guess you could put in a new staircase within the extension from the family room just up to the new master bedroom.

Thank you. We’re not too bothered about losing the two en-suites. They need replacing anyway so wouldn’t mind chopping that space in half to make a corridor to the new upstairs room. We only really want an en-suite for a master bedroom plus a main bathroom. We just can’t think of a way to avoid a dark corridor in the middle of the upstairs!

It feels like converting garage to games room, accessed via where stairs currently are , reconfiguring the stairs so you can add an extension above garage and utility to give you the same footprint for a master and ensuit/ dressing room; my be a more practical solution. Alternative would be to reconfigure a lot more walls internally before you can add an extension to the other side of the house. Another option would be loft conversation to create a games room at one side and guest room on the other side. Then convert the bedroom next to master bedroom into master ensuit/ dressing room.

Thank you. We’d be open to reconfiguring walls inside but we don’t want to lose the garage. There would be room to build another one but we also don’t want to lose them front garden area. Do you think it’d be possible to add stairs off the existing stairway to access a room above the garage? Also, I don’t know what’s involved in underpinning a garage to build above it if thats needed. Do you end up with steel supports taking up lots of space?

Depending on the age of the house, and the actual construction of the garage, it may be easier to knock it down & restart instead of underpinning if you planned to put a room above it. You’d probably need a structural surveyor /engineer to advise you.

Personally I think that you have to consider three things. Firstly - given you want new bathrooms and presumably kitchens and carpets etc- how much budget will be left for building work? this will in part inform your plans.

Secondly - where is the best place to build?- personally I think you need to consider if the side garden has the best privacy or light and if the overall garden and the windows that benefit from it will be better for building elsewhere. You also need to think about how you intend to access additional rooms and if your current lounge becomes a cut through to another room does that make that room redundant/ feel smaller/ difficult to furnish.

You also need to think about the size of the space you have now and how it works for you- so if you make the overall house bigger will the kitchen and dining room feel undersized? Is the idea of the games room to have a family living space in which case it will be best connected to the kitchen or is it that the kids are gamers and a room for that could be in the loft or over the garage or in an underused guest bedroom? Since the first improvement you mentioned was the bedroom would you prefer this to be at the back?

It would be useful to have more context and an idea of the size of plot and orientation of the plan.

Just to show you that extending sideways isn’t the only option

Thank you very much that’s really helpful to see. We could probably extend at the back without needing planning permission too, it’s just that there is lots of space to the side of the house which seems the obvious space to use, but maybe we’d get a better layout going out the back instead

My advice is to get an architect, tell him how many rooms you want and what’s important to you. Then his job is to figure out the best design and also get you light. Roof lights and light tunnels always viable options, again get an architect.

I'd use the bedroom above the front door as the dressing room through which I'd access the main bedroom over the double garage with its ensuite above the loo that's at the back of the garage.