There are things your should consider before you start planning for a secondary structure on your lot. The design and planned finishout will have some consequences you might not have thought about. Many homes in this area have secondary structures so we have a good handle on what you can expect depending on the design of your building. It all comes down to this ...
Will the two structures be connected by the same roof?
If not, you can almost always add a secondary structure on these one acre plus lots as long as it isn't intended to be another home. Garages, workshops, and barns are usually fine as long as they meet the subdivision guidelines.
The workaround for this is homeowners will build a two story secondary structure, maybe filing the permit for a garage. Once the structure is completed and inspected by the city, they finishout with HVAC over the garage area and it's not a big deal. What these cities don't want to see are two complete homes, upstairs and down with bedrooms, bathrooms, kitchens, etc.
If the answer is "yes, will the connection be an open breezeway?
This is now considered one structure and there's usually no issue with finishing out the secondary structure completely as livable area. The home would have to meet the subdivision guidelines but it's rarely a problem.
The issue you might have here, depending on the city, is you have just increased the area under roof significantly. If the area under roof exceeds any new ordinances regarding sprinkler systems over a certain size, you might be required to install one.
Will it be an open breezeway or enclosed area with heat and air?
This is where you might start running into problems. Most of these towns have adopted ordinances stating new homes cannot exceed a certain number of square footage under roof without an interior sprinkler system. I'm not talking about airconditioned, livable area, but area beneath the entire roof including covered patios and garages.
Homes built before these new ordinances were adopted are grandfathered and don't have to comply. But adding a secondary structure with a continuous roofline will require a building permit. When you file your permit and the city sees your new square footage under roof exceeds their new maximums, you could very well be hit with the cost of installing a whole house fire sprinkler system.
Summary of the Pros and Cons of Connecting the Main Home and Secondary Structure with HVAC
The upside - Obviously, the homeowner doesn't have to walk outside from one livable area to the other. And when they're ready to sell, they also get to show the total livable square footage in both structures on the MLS.
The Rule of Thumb with the MLS is if you can walk from one livable area to another without leaving the HVAC, it's considered one contiguous living space. If not, the square footage has to be entered as two different living spaces
The downside - There's a very good chance the homeowner will have to install a whole house sprinkler system in both the main home and the secondary structure because they have combined the two livable areas into one.
I asked Karen Beaty, our "goto" appraiser for her opinion on attached structures with finishout. She said "I see them all the time. It can can really be disappointing when we go out to do an appraisal and the house shrinks from what was shown on the MLS". If the MLS says 4,000 SF she expects to see it but agents often combined two unattached living areas into one and it's a bit misleading.
Fred Bivens, a "by the book" home builder and excellent remodeler told me there are many ways to comply with city regulations and still meet the homeowner's needs.