Wow! I didn't expect that many responses in under an hour. Here's some more information based on the questions.
The cars are all relatively small, wife drives a Fusion and I have a Focus (smaller first generation). Although we may have to get something larger, since the Fusion is getting small for two kids.
I will consider the 18x8 door for easier parking and to accommodate taller vehicles. Is a 20x8 door a common size?
The size is restricted to roughly 900 sqft by the city based on a percentage of house size. This would allow up to a 24x36, but that might be overkill for my situation. Plus I currently have a 14x20 one car garage which will become extra storage and space for small projects.
The new garage will be setback about 10 feet of the back alley, so I should have room to maneuver vehicles.
You have small cars now, but as i told all potential clients, you owe it to yourself to think long term, and expect the unexpected. most garage builds we did were for guys that liked to tinker on their own stuff in one way shape or form...very few we ever did for a skill-less soccer dad that held a desk job and used a garage for nothing but storage...so if you can swing the budget, you are always better off to go as big as you can go within your alloted budget, or at least during construction design it for future addition so it will make the job faster/easier when that time comes and you have more free budget.
I understand the 900sqft thing. Here we have basically 1 major rule zoning looking at first and foremost on the site plan and when they pull up your property info...no more than 30% of the property can be under roof. Many times this required us to demo a shed or another outdoor structure in order to free up sqftage to meet the homeowners new garage size they wanted....you can do the wink wink deal and remove that existing single stall garage, or have it moved off site, build a larger garage, then bring it back in as a shed once your signed off on and hopefully it wont be an issue down the road...we have also been able to, in a single case we had like yours, demo the concrete floor in the single stall garage and bring in rock....they, the city, then reclassified it as a movable structure and did'nt count that as area under roof which allowed us to increase garage size...food for thought.
20'x7 or 8' are NOT common, they can be special ordered obviously...16'/18'x7 or 8' are common doors and often stocked at home centers. I can tell you with all honesty, before committing to a homecenter OHD where your purchasing materials from, you can often UPGRADE to a much higher quality door from an actual OHD company. We used to order OHD with our packages and install them ourselves, then i got connected with an OHD company and for $150 i got a much higher quality (thicker guage metal/better R value) than the "5 star" OHD's we had been pruchasing through homecenters...and the BIG KICKER, they only charged me $200-250 per door installation...I could'nt put my guys on it for that, so I just started specing and subbing out that aspect since it gave the customer an even better product for the same money.
Like I said though, find out what your city will allow for wall width. NBC has changed ALOT over the years and there is now a minimum wall width we have to abide by unless we want to get into sheeting the wall inside and out with double layer plywood and a specific nailing schedule to meet the sheer code they now have in effect. We've all seen garages with 12" wall between OHD's or at the corners of a building...they are simply not allowed here anymore due to code and it forced us to redo man door layouts to achieve minimum wall widths for code, or an increased cost for structural interior/exterior wall sheeting/nailing schedule...you absolutely do not want to just build what you want only to have an inspector come by for the final and say "these walls are too narrow, you have to do x,x,x,x,x before i will sign off....because each one of those requirements will be VERY time and VERY budget unfriendly.
