An accessory dwelling unit (ADU) is a separate living space on your property. It can be a small outbuilding that’s not connected to the main house or might be an expansion of a central building. An ADU can serve as a long- or short-term rental unit, a personal gym or an art studio. It can also allow aging parents or adult children with disabilities to live closer to their families while retaining independence.


There are many things to consider if you’re constructing a new property. How do you prioritize your checklist when building your dream ADU?


1. Consider the Purpose

Before you call a contractor or start shopping for furniture, ask yourself why you’re building an ADU and consider the ramifications of each reason. For example:


    • Your aging in-laws are moving in: Many adults find themselves in charge of a parent’s care and must think about accommodating them. Do you need to install a wheelchair ramp or walk-in shower with handrails? Will your parents live in the ADU all day or just sleep there at night?
    • You’re building a personal gym: Will you need extra outlets to handle multiple exercise machines? Where will the air vents be positioned and is there enough airflow to keep you cool while working out?
  • You’re creating a long-term rental: Will you need a separate electric meter? How much will you rent the property for? Will renters want additional privacy measures around their ADU, such as trees or a privacy fence?
  • You’re building a bed and breakfast: Decide if you’ll allow pets. Will linoleum be easier to clean than white carpet? If guests steal or break a light fixture, how easily can you replace it? If you live in the city, will guests have easy access to parking?


Additionally, think about the size. There are many benefits to designing a lower-scale home, such as lower heating and cooling bills, less space to clean and a reduced build cost. Smaller may be better if you’re planning to rent out your ADU as a weekend getaway.


2. Research Any Regulations

In San Diego, there are four recognized types of ADUs:


  • Conversion: These are new units that convert space in a residential building.
    • Attached: These expand a residential building.
  • Detached: If you build a new free-standing building on a residential property, it’s considered detached.
  • Junior: This is where you convert up to 500 square feet of space in a single-family structure. It requires owner-occupancy in either the remaining portion of the single-family home or the ADU.


The type of ADU you can build depends on your building type and the ADU Program. Go to the San Diego planning website to learn about regulations and building codes. 


3. Estimate the Cost and Get Approved

Depending on their size, the building materials you choose and where you live, additional dwelling units range dramatically in price. Break down the expected costs on a spreadsheet and see where you may be able to reduce spending. While thrifty shopping feels great, you should invest in solid foundations so your ADU will be structurally sound.


You’ll need an estimate of how much you might spend to get approved for a loan or building permit. Once you have a ballpark figure, reach out to a mortgage lender or credit union to get pre-approved for a loan.


4. Choose Between Prefabricated or Stick-Built

Stick-built ADUs are constructed on site from the ground up, while prefab models are made partly or entirely in a factory by a single company and delivered to your property. They tend to be similar in cost, although prefab options are usually faster to put together.


5. Hire a Designer

You’ll need a designer even if you choose a stock design. You can choose between an independent designer or a company specializing in ADUs. Ask them to meet to discuss your portfolio and design ideas.


6. Select a Design

First, someone will visit your property to measure the site for the ADU and locate water, sewage and power lines. Then, you can start designing.


Choosing a stock design is cheaper than designing a building from scratch. This may not be possible if you’re constructing a custom add-on to an existing structure, but it’s worth considering if you’re designing a detached ADU.


You can also cut costs by slightly reducing the square footage of your ADU, using drywall for the ceilings rather than wood paneling and opting for cheaper cabinets.


Regardless of your budget, a few things you should invest in are a solid roof, double pane windows, plenty of insulation and high-quality appliances. You’ll save money in the long run by not skimping on those.


Keep in mind that newly constructed ADUs require solar panels in California, so factor those into your budget. Also, consider whether you’ll need a new electric meter.


7. Submit Your Design

To get permits, you must submit your construction plans to a city planner who will review them. If they ask for revisions, your designer will edit and resubmit the plans. The building cost might change slightly in the process. Once approved, the design is final and you must follow it to comply with the permit.


8. Hire a Contractor and Start Construction

It’s finally time to watch your dream ADU start taking shape. As contractors get to work on the construction, a few unexpected costs can pop up you should consider building into your budget.


For example, some estimates a contractor gives you only include the bottom line price and don’t include things like fixtures and appliances. Additional site work may be required, such as leveling the site or removing trees and rocks. Installing plumbing and utilities can cost more than expected if the contractors have to navigate around underground rocks.


The cost of materials can fluctuate between the time you designed the house and when you actually bought the supplies. Finally, pad your budget for decorative touches like tile backsplashes or baseboards you may decide on at the last minute.


9. Have the ADU Inspected

When you finish the construction phase, a home inspector will review the additional dwelling unit and ensure everything’s up to code. They’ll check the building’s plumbing, wiring, structural integrity, smoke alarms and more. If you pass the inspection, you’ll receive a certificate of occupancy. That’s the final approval you need to move into your ADU.


Enjoy Your New Space

When the sawdust has settled, cabinets are painted, permits are in order and the last screws are twisted, take a moment to breathe. You did it! By prioritizing your checklist, you were able to build your dream ADU.