What do you think of this layout?

[GIK Acoustics]

You don't necessarily have to do a coffin shape, though angled walls help to give healthy reverb - so if recording and mixing in the big room, I would certainly suggest angling both the long sidewalls (6 degrees, each). I would personally rather opt for the large room as the tracking and control room, while the small one could be an ISO as you suggested. 8x10 is pretty small for a control room, but the dimensions in either room aren't very great either so both will have their problems. Do you have a budget for acoustic treatment?

[Jesser]

Yeah, I will see if we can get those walls splayed out. I'm aware that a better ratio for the big room would be about 3 feet less wide with a 7' ceiling. If I can get him to do the angled walls, we can easily make the room skinnier. What are your thoughts on the all-insulated ceiling?
As for treatment budget, I already have 3 GIK broadband panels and would probably purchase either an additional room kit or 3 more panels w/ DIY bass traps. Was thinking I would probably build a DIY diffuser for the back. Hadn't thought as much about treating the smaller room yet.

[GIK Acoustics]

A lot of people go for a really large cloud in both mixing and tracking areas, so I think the open ceiling will be very helpful for your room. If that is the case though, I would avoid carpeting the floor as it would be mostly unnecessary. You could use thick rugs wherever necessary to avoid reflections from the floor (say, below the drumset) but having a fully absorptive floor and ceiling would kill a lot of the rooms reverb.

The smaller room will likely need to be more dead than the larger room...natural reverb from an 8x10 room isn't the most pleasing sound to have glued to a recording.

[Jesser]

Revision. Should I conform these rooms a little better to proper ratios and "waste" space? We've talked some about extending the loft floor in that 5x10 space and making the stairs a straight shot on the bottom corner there. Would make this whole deal a lot more flexible in my opinion.

[Jesser]

Here's the new floor plan. We were able to extend the floor to gain precious space for the control room. I've marked light sconces and outlets. The orange outlets will be near the ceiling and on the light switches to be used with strings of christmas or rope lights. The green rectangle represents the XLR panel. Treatment in the control room will be a GIK room kit +three extra broadband panels and a DIY diffuser. Live room will be DIY Roxul traps and DIY Owens Corning panels.

[mattbow1990]

Looks like a good layout.

[GIK Acoustics]

Looks much better - a good use of the space certainly. Keep us posted on how it turns out!