Do you mean that you're converting the playroom into a kitchen diner, and want ideas for what you then do with the garage? Or are you converting the whole ground floor into a kitchen diner and want ideas for that? Not sure if you're looking for layout or decor ideas?

Layout ideas please! We don't really know where to start. I would like to put the kitchen downstairs and ideally a utility room too. Then convert the old kitchen into the master bedroom

The garage door has to stay so the outside is the same but could change the layout of the ground floor

I would probably do something like this.

As you have to keep the garage door I would block off and keep a mini garage (useful for bikes etc) this could also house utilities.

then open up the rest as much as structurally possible. putting the dining room in the darkest section and kitchen can go under your existing to make drainage and wiring all a bit easier (cheaper!)

Great idea from becci that I can't improve. The only thing I would add is in the dining room you could do a built in banquet with pendant lighting over the top, make it quite atmospheric as it's the darkest part. Or, depending on how much width you have, as it's the darkest area you could use this space for your sofa/ tv and then put the dining table behind the island