Without seeing the building and knowing dimensions, it's impossible to respond.

Leave it open to maintain a more airy feeling.

Are you wanting to add a Loft to an "existing" Garage Apartment?Check with Building Department regarding the "use" of secondary dwelling unit. If its all good, you might be able to lift a portion of the roof with roof dormers to gain ceiling height? Best to upload a few pics as suggested in post above.

Kevin's right, you need to check with the local building department to confirm that a secondary dwelling unit is possible. Then I would strongly suggest you seek professional assistance with your plans..........just about everything from the foundation up needs to be reviewed to determine that you have the capability to add the new loads that a loft would impose to the entire structure............if you're going to do it, do it right the first time........doing it twice is way more costly. Good luck.

Words of wisdom...

Been there...done that! Best to measure twice and cut once! It's all a big learning curve, best to seek professional advice, money spent up front saves in the long run!