When can you add an ADU?
Not all homeowners are eligible to build any sort of ADU addition. Whether an ADU will be allowed is determined by local zoning ordinance. There are all sorts of constraints that may impact whether/how your property can add on an ADU. These include but are not limited to:
- Age and size of pre-existing residential structure.
- Conflicting zoning (e.g., other ground-floor spaces in the neighborhood are zoned for retail).
- Water availability.
- Septic system -- can the existing one also serve the new addition?
- Minimum and maximum height requirements in relation to existing structures.
- Whether your ADU will remove existing parking spaces.
Keep in mind that if the local law does not think you should build an ADU on your property and you decide to anyway, you are most likely violating the building code and may find an official paying you a visit and demanding that you tear it down and/or pay a fine.
Accessory dwelling unit permits
All ADU additions need to be treated as major residential construction projects, meaning they will need to be permitted at every step. The first crucial step is getting your overall ADU addition plans approved by your city. Additional to the basic building permits, you may also need an encroachment permit, electrical permits, and a variety of other permits. Pulling these can entail substantial cost. Once you have the permits, construction can commence. Then you'll also need to get all finished construction checked off as being up to code in order to be compliant.
Different states, and different municipalities within states, have different laws that govern ADUs. Generally, if a state decides to implement a policy allowing for ADUs on single-family residential lots, as California did, that regulation becomes the minimum baseline. Cities can expand on what's allowable, but they can't rule against the state and disallow. So check with your city and then your state before approving a building plan. This site provides a starting point for researching other cities, although it is crowd-sourced and not 100% maintained across all states.
Cautions against adding an unpermitted ADU
There's a contingent of homeowners and even some real estate and residential construction industry folks who don't see the harm in converting the garage or other constructing some other junior ADU unit without getting the proper building permits. The logic tends to be that this is work you're doing on your own property, and whatever happens inside one's own four walls is one's own business.
This is only true, however, until and unless something goes awry in the new construction. If unpermitted modifications to electrical wiring or plumbing result in an incident, such as a flood or electrical fire, the insurance won't cover anything found to be caused by unpermitted work. And likely it won't cover areas that were built unpermitted in any event, even if there's no proof the problems stemmed from the unpermitted work.
Pros and cons of having an ADU
If you get an ADU that's well designed and built and within your budget, the biggest thing to check off in the "pro" column is that you have more livable space for family or renters. ADUs do not necessarily add much value to a property, and they can be fairly expensive and time-consuming to build.
That said, a nicely finished and furnished ADU on a desirable property can often be rented for slightly more than the value of comparable apartments. For a renter who wants to be in a certain neighborhood, finding such a rental means gaining access to a place where standard-size single-family homes are too expensive or unavailable to rent.
And from the community planning standpoint, ADUs allow for a way to increase housing density proportionally to the amount of single-family residences, allowing homeowners to share in the increase and the income without making major zoning changes.
Still worth it? Most people say yes
Although building an ADU addition to a single-family home can be costly and time-consuming, their popularity is evergreen and continues to rise, especially in areas with strict residential zoning and little housing availability. While we recommend staying on a strict budget and planning things very carefully, homeowners who follow these measures typically find ADUs to be a worthwhile long-term investment in a family home.