Before you worry about the technicalities, make sure there is a solid business out there. Who is your target audience? How will you reach them? How much would your service cost? Keep in mind that most people who want their place to be a workshop have less money, and more time. AND they are DIYers who are more likely to do the work themselves (albeit slowly). Those with money to hire you are less likely to want to WORK in a shop, and more likely to want their garage to be more man cave and less work space.
That doesn't mean there isn't potential, just make sure there is plenty of work before you jump into it too deep.
I'm a General Contractor here in North Carolina, so take this with a grain of salt. As with every business you need to find your customer and I believe you will find that the demand is very low for custom garages. I have built homes from 89k to 1.25mil, and the garage is the last thing most homeowners put any thought into. Most could care less about other features other than a automatic door opener. With home sales and home values in the toliet, I think it would be a tough sale for homeowner to justify a 25k cosmetic upgrade to the garage.
I think the custom garage person is a DIY type guy who would more than likely handle this him/herself and leave the contractor for the full blown kitchen or basement remodel. I have thought about this business model but can not see the demand to justify license, insurance, overhead. Just my thoughts...
Good thoughts go before me.
I am a general contractor as well but what's more is that I have been a specialty contractor for most of my 40 years in the biz. I could write a book, but I'll try to summarize some points:
Target market, yes sir. You need to know this aspect of marketing more than anything else. I'd try selling for another entity before launching any business that will cost you any capital to set up. And what serious business doesn't require capital investment?
Sub contract vs. in house. Either way, you need insurance and GC insurance is the most expensive. Don't leave home w/o it.
People. Well, they are picky and then some. Even though it's only a garage, they will examine the work like it was a trophy kitchen. Be prepared. Oh, and the checkbook always seems to be out of reach. You need to have really good agreements and stick to them with no give aways. Even at that, you will give away a lot of time. And there is a time to do that for goodwill and all that, but you have to stay business like from the get go or you will be had time wise.
Planning and salesmanship. The ability to produce the job in a 3 dimensional laptop program for presentation purposes will save a lot of time in planning, changes and, worst of all, in progress changes. You will want to print out plan view and wall elevations also stating all details, brands, level of finishes, everything. You can't be too detailed. They may only want to skim over some of this, but beware of that and get initials on every page.
I could go on but I'll wait and see what others have to say and comment accordingly.