Several images may come to mind: an apartment over the garage, a stand-alone building in the backyard, or maybe a converted basement. Known officially as an accessory dwelling unit (ADU), mother-in-law suites have many styles and almost as many names, including accessory apartments, guest house, carriage houses and granny flats.

One thing all ADUs have in common though is the potential to increase the value of a home. Of course, the main reason homeowners decide to build one is to either create an opportunity for rental income or provide living accommodation for extended family members.

It’s easy to see why a homeowner would want to build an accessory dwelling unit. Elderly relatives can live near their family as they grow old and infirm. Adult children get affordable accommodation while they’re saving up for their first ‘real’ home. Tenants offer an additional income stream. These benefits often outweigh the cost of the construction, the increase in property taxes, and the necessity to comply with zoning regulations.

However, be warned. Not all types of ADU are allowed in all counties.

In St Johns County, Florida, accessory dwelling units are permitted, but under strict conditions. The main restriction is that you cannot rent out your mother-in-law suite on a commercial basis. So, before you draw up your multigenerational floor plan, make sure you’re familiar with the county’s building permit requirements, which I’ve set out below.

To get your mother-in-law suite or granny flat classified as an ADU (and not simply an extension), it must have its own kitchen, living area and separate entrance. Typically, the structure uses the same water and energy connections as the main house, but it can be attached, detached or internal. But even if it’s a detached unit, you cannot sell it separately from the main house.

In St Johns County, the construction of mother-in-law suites for non-rental purposes is generally permitted in all residential and/or zoning districts.

In other words, you—as the owner of the main house—can allow relatives to reside in the unit, but you cannot let it commercially to people who are not related to you.

Below is a summary of St John’s County zoning requirements for mother-in law-suites (what the county calls an ‘accessory family unit’).

An Accessory Family Unit:

• is limited to family members of the owner(s) of the main use structure
• must be non-rental and cannot be sold separately from the main use structure
• must be located on the same lot as the main use structure
• may be at least 800 square feet but it must not exceed 35% of the square footage of the main use structure or up to 2,000 square feet of living quarters, whichever is the lesser
• must offer permanent access from inside the main use structure if it’s an internal structure
• must meet the same required yards as the main use structure
• must provide adequate parking
• must comply with all other zoning and permitting requirements

Additionally, if a manufactured or mobile home is used as an Accessory Family Unit, the lot must be at least two acres and zoned for the use of a manufactured or mobile home.

As long as your mother-in-law suite or granny flat meets all the above requirements, it won’t be considered a separate dwelling unit for the purposes of the and Comprehensive Plan. In other words, it won’t be subject to a separate property tax assessment.

If you’re thinking of adding an Accessory Dwelling Unit to your property in St Johns County and would like a professional custom floor plan drawn up, please contact me.