Chris D. Craiker, The Napa County Architex Angle: So you want to build an ADU… One-story or two? – Napa Valley Register

The recent showcase of ADUs, accessory dwelling units, put on by the Napa/Sonoma ADU Center highlighted how homeowners and builders can tackle creating granny flats in one’s backyard.

The examples range from 150 sq. ft. to 740 sq. ft. The construction types varied from conventional “stick-built” to factory and partially factory manufactured products. There will be many ADU plans available, including many municipally preapproved plans by some government agencies.

The center has an informative brochure, www.napasonoma.org/workbook I found it very impressive and quite accurate, excellent for owner or architect. It notes that building smaller isn’t necessarily cheaper per square foot. One will still need a kitchen and bathroom, plumbing, electric service, a foundation, which doesn’t get necessarily cheaper when smaller. And the processing is every bit as laborious for 150 ft. as a 1,200 ft.

One of the most important sacrifices a homeowner has to make is land area. One’s yard will be reduced by up to 2,000 sq. ft. with setbacks, private yard area, parking and access. While the local codes do allow reduced setbacks, building an ADU, no matter how small, is a big personal sacrifice. One could convert an existing garage, which is allowed, but construction cost is not cheap, and one will lose the parking or storage option

Building over an existing garage can be expensive since many old garages have poor foundations and weak structural walls. By the time you’re done reinforcing the existing foundation, strengthening exterior walls and building a stairway it’s probably cheaper and more efficient to start from scratch.

Many homeowners ask me about building a ADU above a new garage. In most municipalities a second story requires design review for protecting privacy and solar access but is generally allowed if one works with their neighbors. If you have the land area this can be a good solution for many homeowners. The question is always, “What’s the cost difference?”

There are so many factors involved. For basic ADU construction, we “guess-timate” a general budget be between $300 and $500 a square foot. Again, a budget not a fix price. Let’s assume for a moment, one wants to build a basic, efficient, one-story, one-bedroom ADU with a bathroom, kitchen, laundry facility and living/dining room ADU at 499 sq. ft., for minimum municipal fees. If rectangular and compact, that can also be built on top of a new garage of approximately 22 by 23 ft. size. What is the difference in costs?

In theory, the roof, and foundation, and most of the internal parts of the ADU will be pretty much the same, although two-story support walls as well as the foundation will require more reinforcement, thus adding expense. A structural second floor will be required as well as lights, fire protection and a garage door. And we mustn’t forget there are stairs and probably a balcony of some sort for an outdoor sitting area, hopefully not waterproofed.

I compared the relative construction cost for three ADUs we designed on flat land and three built on top of garages. Their finishes, square footages and site work were all different, so a scientific analysis is not possible. The cost per square foot of living area ranged from $357 to $650 but the units over a garage cost approximately 27% to 40% more per square foot for living area only. If factoring in the new garage square footage, the cost per square foot is far less.

Definitely, the two-story garage ADU will cost more, but in the long run, one will gain a two-car garage with storage. One way to look at it is: a 500 square-foot ADU will cost X and a 500 square feet ADU over a garage will cost “X times,” perhaps 33% more. That’s 133% of building an ADU on flat land. Definitely unscientific.

What’s may be more important is who is going to occupy the unit. A senior or elderly person may not like stairs, and privacy between neighbors will be critical. A two-story structure may require approval by the local planning department. While fairly common, an important issue is protecting privacy between neighbors while not reducing solar access.

Building a ADU over a garage is not free but you’re essentially creating land by building a garage that can house cars, workspace and storage. It’s all a matter of personal priorities.