Some of the challenges listed below have been addressed in the new California state law known as AB68.
There are often zoning restrictions on accessory dwellings. While the state of California forcefully encourages jurisdictions to make it easier for home owners to build accessory dwellings, most still have some limitations. Check with your local Planning or Zoning department to see if accessory dwellings are permitted in your neighborhood.
The answer will be “no”, “yes”, or “yes but”. The last category will usually be a limitation on your lot size, set-backs, or something similar. Although, one Peninsula jurisdiction limits how many accessory dwellings were permitted by saying you could build one IF there wasn’t another such dwelling within 400 feet.
For the in-law units within existing houses, especially in San Francisco, you will be required to meet current building code. The current code says that the minimum ceiling height is 7’-0”, which can mean some expensive excavation and new foundation work if you can’t meet that requirement.
Note that San Francisco has amended the building code to require a minimum ceiling height of 7’-6”. San Francisco is the only jurisdiction in the Bay Area that does not use the California Residential Code. San Francisco also has a significant amendment to the building code.
Each jurisdiction deals with the allowable proximity of accessory dwellings to property lines differently. In some jurisdictions, they allow the accessory dwelling to be closer to property lines than the main dwelling is allowed.
For example in Oakland, or Berkeley, the accessory dwelling can be built as close as 4 feet from the property line with some restrictions. Other jurisdictions set that number at 5 feet or 10 feet.
In Sunnyvale, the ADU can be built 5 feet from a side property line, and 10 feet from a back property line.
In San Francisco, with most houses built right up to the property line, accessory dwellings are allowed to be built within the existing envelope of the house with the walls at the property line having a 1 hour fire rating. The walls and ceiling/floor assemblies separating the new accessory dwelling from the adjacent spaces also must have a 1 hour fire rating.
Dealing with utilities for the new accessory structure is perhaps the question with the most different answers between jurisdictions. Some allow the main house and the accessory dwelling to share water, sewer, and power. Some require that one or more of these utilities be separated.
Bringing in new utilities, even to a property where the utilities already exist can be very expensive. Various utilities charge $10,000 to $25,000 for new utility hook ups. You need to check first with the building and/or planning departments to see what the city allows/requires, and then check with the utilities to see what the new service hook ups/installations will cost.
Parking can be a project killer in some locations. Most cities will require at least one, new, off street parking spot, sometimes covered, when you add an accessory dwelling. In California, if the property is located within ½ mile of public transportation (bus, train, light rail), then the parking requirement is waived.
If you aren’t so luck to have public transport within ½ mile, you will be required to provide parking. The parking limitations will vary from jurisdiction to jurisdiction.
Out Door Space
Some cities, like Berkeley, will require that each dwelling on the property have at least 300 square feet of dedicated outdoor space. This space cannot include paved paths or driveways, or the front yard. On smaller properties, with the single story limitation in Berkeley, the allowed maximum size can rapidly be reduced by this out door space requirement.
San Francisco has an “exposure” requirement. Exposure means that the accessory dwelling must have windows that face out onto the street, or onto a side or back yard at least 20×20 in size. The Zoning administrator may waive that requirement down to a space 15×15, but it will depend on the configuration of the lot and adjacent buildings. The purpose of this requirement is to ensure that the accessory dwelling have sufficient day light, and not have it’s only windows opening up onto light wells or narrow side yards.