Thanks Edward, nice of you to notice
We have put a fair bit of thought into the layout before committing to the contract.
I'll keep adding updates as soon as I have something useful to say At the moment, we are halfway through the DA stage and there is not much to report.
Regarding the two areas you mentioned:
Under the stairs
The plan here is to put our IT stuff. Modem, routers (wired and wireless), patch panels, NAS units with additional external backup drives. All the sort of things that you want access to but don't need to have sitting out on benchtops all of the time. For the main wireless router servicing phones/tablets etc I will possibly push through a wall and mount it either on the lounge wall or on the stair wall - if signal strength is not ideal under the stairs itself. If it doesn't seem to be a problem then I'll just leave it under there.
Garage
The plan here is to keep two cars (in comfort - not opening doors onto the other one all the time) as well as motorbike, trailer, possibly a small boat and then all the "toys" that a family can accumulate (bikes, skiis, snowboards, climbing gear, tents etc etc etc) but in a way that they can be easily accessed without digging through a cupboard of "stuff". We have a mental image of being able to go on a skiing holiday and just going to that part of the garage, pull out all of the stuff we need in one go and put it in the car. Same idea for "going climbing" etc.
Also a wall for tools and then the garage can be turned into a workshop by moving cars/trailer out.
Finally, the space becomes flexible for children's parties or other shenanigans (when the weather turns nasty) by again moving things out and creating a large space with large double door openings at each end.