Okay, maybe way too much about the house, but I've been thinking about space design myself recently, so here goes:
The big common room: This room is going to be awfully noisy. Basically, anyone who wants to watch tv/listen to the stereo or radio is going to be competing with everyone else. I know that at least for myself, I really really like silence in the mornings when I'm eating breakfast, while someone else may want to watch cartoons. Similiarly, I hate having to listen to tv shows I don't like, so it would drive me crazy if I had to cook dinner and couldn't block out the noise. I'd guess the idea here was that people could do different things while still being in the same zone, but TV doesn't share well with others. I'd suggest instead making a family/tv room with big doors that can be opened into the rest of the common space when people want, and shut off when other people want quiet.
Kitchen: That's a one-butt kitchen. Yes, technically you could put two people in there, but even when they are at different counters they are going to be impinging on each other's comfort zone, and if they step back at all, to look at what they are doing or to reach for something, they are going to be slamming into each other, annoying at best and dangerous at worst if people are carrying hot food. I'd double the size at an absolute minimum, and even then it'd be a very small kitchen. And a walk in closet to use as a pantry is really, really nice.
I'd suggest putting the breakfast bar in between the kitchen and the main room. This marks off the kitchen but allows free conversation between people in it and the main room, saves wall space in the main room, opens up the back of the breakfast bar to be used as kitchen cabinets for storage, and allows the bar top to be used as extra counter space when not in use for eating. And it is nearly impossible to have too much counter and cabinet space in a kitchen.
The studio: it seems odd to me to not have a door between that and the main hall if you intend to use that for recording.
The office: that's an awful lot of space to use for an office; you rarely, rarely need that much space for actual work, unless two or three people need desks in there. In fact, it can be a disadvantage to constantly having to move around that much to reach stuff. I'd cut it in half and go vertical on storage. The one exception: if you have clients coming in and you want to have a place to schmooze with them, I'd turn the part of the office that is closest to the main door into a seating area where you can do so.
The instrument storage room: The problem with having a 'storage room' is that it tends to be dead storage; people rapidly start chucking things in there and closing the door, and pretty soon it's a solid mass of Stuff. Now, dead storage is not a bad thing; I love attics where you can put things away for 40 years, or lumber rooms in the basement where you can put everything you might ever need to use again. But in a 900 foot house, that's great deal of space to devote to dead space. I'd suggest instead active storage. For example (I don't know if this would work acousticly) if you make the studio space bigger and then lined the walls with cabinets from floor to ceiling, appropriately set up for instrument storage.
The guest room: If there isn't a door between it and the and the family entry (and it looks like there's a solid wall of stuff blocking it), there should be. Right now a guest taking a night time pee or coming back from a shower has to pass through the living/dining area and the main hall -- quite a parade if you are in a towel or bathrobe! Also, if you have a professional area of the house, this way you can have a work zone and a family zone, and people wandering to the library to find a book don't run into your clients.
To go with this, I'd move the door from the guest bathroom to open into the mudroom instead of the main room. To be blunt, the process of using the bathroom sometimes involves smells, and you don't really want those smells (continued...)