Garage Apartment Plans/Ideas?

Before you expend time, money, and effort on worrying about layout and interior design, first 3 steps are to make sure the basic structure there meets local code for living space (floor strength, fire breaks, egress requirements, that sort of thing), that there is a practical way to get utilities (plumbing and HVAC) up there, and that local zoning and the covenants for your neighborhood or subdivision (if they exist), allow for rental units and/or 'granny flats', aka 'mother in law' apartments. For example, on the egress, most of the areas I am familiar with require an exterior stairway, and most areas require a full fire break between the garage area and the living space.

If none of that is an issue, like you said, it is a site specific thing. If you don't have the expertise yourself, spend the money for a site survey by a residential designer or architect. A good one will have the expertise to answer the engineering questions, and have a grasp for what is needed to make a livable space that someone would actually be willing to pay rent on. Even if this is for guest or family use, think about who you envision sleeping up there- will they be willing/able to deal with stairs, especially in bad weather? If they are older, how about in five or ten years? A glorified tree house for a boomerang adult child to use for a couple of years is one thing, a safe place for a mother-in-law is something else entirely.

aem sends....

While all those are good caveats, I don't think you necessarily need an architect/engineer to desing the place, unless your municipality requires stamped drawings.

Find a website on which to stick measured drawings and a couple photographs each of the inside and outside, along with an order-of-magnitude guess as to how much money you've got to spend. (Raising the roof is going to cost a bit more than adding a skylight...) and then ask again.

--Goedjn