You might like the idea of having an extra space outside of your house to use for storage, an office, a studio or even a guest house. (Guest house is a great phrase, isn’t it. Sounds so…rich.)
What you may not realize is that an ADU (accessory dwelling unit) is not simply a garage with an unfinished space above it. It must be designed and built- from the foundation up- as a fully viable living space.
What does “viable living space” mean?
It means that you must create a foundation that will support a heated house. The base for a garage typically is just a floating concrete slab. However, the foundation for an ADU must be frost protected and insulated. For an ADU you will insulate out from the slab so that the cold doesn’t get underneath the slab. The interior of the garage will need to be insulated and sheetrocked as well. This adds to the time and cost. Additionally:
- You must also make a connection to the sewer which is typically in the street. You must bury that connection below the frost line all the way to the ADU.
- You must also connect to the city water supply, which is also typically in the street.
- A separate gas line must be created, either from the street or from the existing home.
- Electricity will also be needed. You need to either share it from the existing home or get a separate connection from the power line.
- An ADU will usually have its own address, although currently you cannot sell it as a separate unit.
- I you live in an area that has historic oversight such as the HPC (Heritage Preservation Commission) area in Saint Paul, you will also need approval for your design, size and materials.
As you can see, you need to intend from the very start to create a living space when you create an ADU. Retrofitting a simple garage into an ADU would be costly and probably not worth the effort. Planning and building an ADU without finishing it would be lest costly but, there are still rough-in costs that would not be found in doing a simple garage that would make it more expensive. As I am currently building my own ADU in the HPC area of Cathedral Hill I know the timeline and costs for an ADU. Check out my next blog post for exact numbers.