Ten Things You Should Know About Before Building An ADU


If you're a homeowner in California, chances are you've heard of Accessory Dwelling Units (ADU) - independent residences located on the same lot as the main dwelling.

Also known as granny flats, in-law units, backyard cottages, or simply ADUs, these tiny homes can do a lot. They can generate rental income, house aging parents, grown-up children and lower-income families. They boost the State's dwindling housing stock - a factor that paved the way for Government Code Section 65852.2 (§65852.2), which encourages ADU construction.

If you're planning to build an ADU, now is a great time. §65852.2 relaxed some formerly strict requirements and banned local authorities from imposing certain new ones.

That said, don't think building a tiny back house in your yard will be a piece of cake. As with any construction project, thorough planning is the key to success; that's why we prepared the 10 tips below to help you with your ADU project.

1. Check Development Standards to Ensure Your Property Is Eligible for an ADU

Before you design your ADU, make sure that your lot is eligible in accordance with local development standards. Here are some things to consider:

Density. This refers to the number of dwelling units allowed on a property. Make sure that your proposed ADU falls within the density constraints of your district.

Maximum Height. Your city or county can limit the height of new buildings in your area. To receive a building permit, your ADU's height must meet these limits.

Setbacks. §65852.2 requires the distance between a detached ADU and adjacent property lines to be sufficient for fire safety, but your local authority having jurisdiction may impose more onerous requirements. For example, if you are building an ADU in San Francisco, your rear setback could be up to 25% of the lot's depth, whereas in Los Angeles, the minimum rear and side setbacks are often 5 feet. Along with density and heights, you can find the setback parameters in the zoning guidelines of your authority having jurisdiction.

Utility Easements. These are the public utilities that run under or over your property. Make sure your proposed ADU does not limit access to these services.

If you have a public utility easement on your property, your ADU should maintain a certain required clearance. For example, in Los Angeles any new construction needs to be at least 10 feet away from utility easements; otherwise, an encroachment permit is required from the public utility.

Others. There are other factors which may limit your eligibility to construct an ADU. In Los Angeles, detached ADUs may not be built in Hillside Construction Regulation Supplemental Use Districts, and are limited to a floor area of 1,200 ft² and a height of two stories. In San Francisco, there is no limit on the floor area of an ADU, as long as it fits into the lot's buildable area. However, San Francisco does ban ADU additions to buildings that have had owner move-in evictions within five years or other no-fault evictions within 10 years of permit application. Be sure to check with the building department of your authority having jurisdiction for any other restrictions that may apply to your project.

Once you've determined that your property is eligible for an ADU addition, you can start the design process.

2. Find Out If You Need Additional Parking

Before §65852.2, cities and counties across the State required new off-street parking to be created for an ADU. This condition was notably taxing on smaller lots with little space to spare. Now, you need not worry about a new parking space if your property meets ANY of the following:

  • It's within half a mile from a public transit system
  • It's in a historically and architecturally significant district
  • On-street parking permits are required but not offered to the ADU's occupant
  • A car share vehicle is located within a block of the property

If neither of these apply, you may need to plan for an extra parking spot on your property. In Los Angeles, only one parking spot needs to be provided for ADUs that don't meet any of the exemption criteria described above. In San Francisco, ADUs that don't meet these criteria may have the parking requirements reduced by adding indoor bicycle spaces. Check with your authority having jurisdiction to be certain.

3. Plan Site Access to Your ADU

Access is an important factor to consider for several reasons.

Safety is number one. In a fire, the dweller should have an unobstructed path to the street. Consider the walkway - simple precast pavers will provide a convenient path from the street to the ADU without breaking your bank. You should also have a clutter-free passage between your house and property line, so think of alternatives for your trash cans if that's where you keep them.

4. Consider Privacy

How you integrate privacy into the layout of the ADU depends on the future occupant.

If it's your parents, less privacy could be acceptable in the design. Their front door or porch can face your backyard entrance, giving the living arrangement a sense of inclusion and harmony. Window blinds, trees and plants could serve as subtle privacy screens.

If you plan to rent the unit, privacy can be a deal breaker. Imagine a stranger living in your backyard. Would you want to spend your leisure time in their view? Probably not, and the discomfort could be mutual. Consider a side or back entrance for the ADU, with plenty of shrubbery for an added sense of seclusion. These extra steps can go a long way in creating a comfortable living arrangement for both you and your tenant.

5. Understand Your Site Conditions

Is your backyard flat or sloped? Does rainwater pool in parts of it? Is the soil undisturbed or has the backyard been filled in the past? Site works can account for a significant chunk of change on your project; knowing the answers to these questions before you start work can help reduce unplanned expenses.

If your ADU will rest on a slope, you can even the grade by excavating, or support the structure from underneath. With either scenario, consider access. If the ADU's intended occupant is an older person with mobility needs, make sure they can get to the front door without worrying about steep inclines or stairs.

Building on disturbed soil causes damage to the foundation of your building as it slowly settles. If the soil on which you're building was previously dug, scraped or filled, make sure it is replaced with appropriate soil and compacted, per the soils engineer's recommendation.

If there are drainage issues in your backyard, rectify them before construction. The last thing your ADU's occupant wants is water pooling around their unit.

6. Determine If You Can Tap Into Existing Utility Connections

Thanks to §65852.2, ADUs are not considered "new residential uses" for connection fee and capacity charge calculations. Typically, they also don't require separate service meters.

Water can be drawn from the pipes serving your house, without additional meters.

If you're installing a gas furnace, water heater or stove, you can tap into your home's gas lines and connect a new private meter "upstream" of the existing one.

The ADU's plumbing fixtures may tie into the existing sewer lateral, while septic tank connections, if allowed in your jurisdiction, need approval from the Health Department. What's more, your existing septic tank may not have the capacity to handle sewage from additional bathrooms. Older septic tanks may require an upgrade before a connection to the new unit is made.

7. Verify Your Design Guidelines

The design guidelines for ADUs vary between cities and counties. Yet, common requirements do exist, and your ADU's architectural design will be based in part on the California Residential Code. Per the code, units must include kitchens and bathrooms. Ceilings need to be at least 7'6" high, and habitable rooms shall span a minimum of 7' in either direction. A heating system should be installed for the unit; plug-in space heaters are not acceptable.

Criteria governing structural design and material design, among others, are found in the California Building Standards Code. Because local ADU ordinances may vary in their interpretation of State codes, consult with your local building department to get a clear picture of your design standards.

8. Title 24 Energy Report and CALGreen Code - Make Sure You're Compliant

California's Building Energy Efficiency Standards, better known as Title 24, view ADUs as new dwellings. To comply with these regulations, the envelope and mechanical systems of new, detached ADUs must meet relevant design criteria.

California Green Building Standards Code, or CALGreen Code, also applies to ADUs. The code stipulates a range of building standards aimed at reducing the environmental impact of the new building and promoting sustainable construction. The standards state requirements for such factors as site development, energy efficiency, water conservation, material conservation and environmental quality. All California planning departments will have a checklist of these requirements that need to be in compliance with the mandatory CALGreen Code upon submission of your application. Not having these Title 24 requirements upon your initial application can delay approval of your project by several weeks if not months.

9. Know Which Fire Regulations Apply

Check with your building department whether sprinklers are mandatory in your new ADU. Typically, all newly-built units do, but the new ADU statute provides a reprieve from this requirement. Ask the building and fire officials to be sure.

If your jurisdiction does not mandate sprinkler systems for ADUs, you may be thinking about cutting costs and excluding them. If this is the case, keep in mind that sprinklers are an important safety feature in locations where firefighter access is difficult. Having the system can also slash your insurance premiums; contact your insurance company to find out if you qualify for savings by installing sprinklers.

Remember to plan for a means of egress. According to the Fire Code, your ADU should have an unobstructed path and well-defined pathway to the street. If your parents will occupy the unit factor in their age and mobility needs when designing escape routes.

10. Think About Saving Space

Space is an obvious concern for ADU dwellers. With a kitchen, bathroom, sleeping and living space all squeezed into 1,200 square feet or less, things can get cramped. Your choice of furniture, appliances and storage options can go a long way in adding space and comfort to these tiny homes.

Wall-mounted storage is key to reducing clutter, and it works in every part of the unit. Embrace it. Hang the TV on the wall, along with some bookshelves. Fit out the kitchen with upper cabinets and hooks for utensils. Let wall-mounted shelves in the bathroom serve as the linen closet. The more wall-mounted storage options you give your ADU's occupant, the more free space they'll have on their floor.

Avoid bulky furniture. A loveseat can replace a full-sized couch. A Murphy bed can turn the bedroom into a bedroom-slash-study. Chairs and tables can fold to make more room when needed. If your furniture can double as a storage unit, even better. Same goes for appliances. You can save plenty of space by opting for condo-sized stoves, fridges, dishwashers and washer/dryer stacks.

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