How to Build an ADU

The process of building an ADU

IN THIS GUIDE

What is an ADU?

Accessory dwelling units are secondary dwelling units built on the same lot as your larger, primary residence. They’re a great way to add extra space (and value!) to your property.

Certain features “qualify” a building as an ADU:

  1. Its own utility hookup
  2. Its own living space
  3. Its own foundation
  4. Its own water hookup

Types of ADUs

There are 4 primary types of ADUs.

Detached ADU

A detached ADU can be an apartment in your backyard or a cottage. It has its own entrance, it stands separate from your primary dwelling, and it has its own water and utilities. Another person/family could live there entirely separate from your home.

Attached ADU

An attached ADU shares at least one wall with your primary home, and there is no internal entrance to the ADU. The attached ADUs often share utility hookups with the main residence, but they still fully function as autonomous living spaces.

Garage conversions

A garage conversion might be an attached or detached ADU. Either way, it’s a garage converted into a living space, subject to the same four qualifications for an ADU.

Interior conversions

Interior conversions are typically either in the basement or the attic. They aren’t spare rooms; instead, they are fully functional living spaces that operate independently of your house. Although they might not be visible from the outside, they’re still considered an ADU.

Why should I build an ADU?

Before committing to an ADU, you need to know who you’re building it for.

For example:

Is your ADU for aging grandparents who can’t afford a nursing home? Or is it for an adult child still getting on their financial feet? Maybe you’re building for a special needs family member who needs extended care?

Who you’re building for will direct how you approach planning and constructing your ADU.

Aging grandparents need a space suited for limited or reduced mobility, so an interior attic conversion probably isn’t your best option.

An adult child will want autonomy and his/her own space, so a garage conversion or detached ADU might work well.

A loved one with special needs will require accessibility because they might need care during all hours of the day. An attached ADU or an interior conversion would be a good choice.

Calculating the cost of your ADU

You’ve decided what type of ADU fits your needs—now it’s time to start thinking about cost and financing your ADU.

It’s impossible to tell exactly what your ADU will cost. Why? Because of the sheer amount of variables: the structure type, the builder, market cost of materials and labor, etc.

That said, you can examine aggregate data from your market to get a good sense of what you might pay for your ADU. Notably, this data includes (i) design costs, (ii) permit costs, (iii) utility connection costs, (iv) construction costs. It doesn’t include landscaping or furniture costs. This data covers ADUs built in the Portland market from 2016-2019.

Average Cost ($) Sq. Footage  Cost/Sq. Foot
Detached 180,833 676 305
Attached 154,400 556 300
Garage 142,000 504 297
Basement 185,833 676 265

For a deeper dive into the data, check out this post from Building An ADU.

The key takeaway?

The fixed costs (development, labor, materials, etc.) of a new housing development drive the final price of your ADU. As these fixed costs increase year over year, so will the price of your ADU.

The good news? The land you’re building on is free—you already own it! Land typically represents the most expensive part of any real estate project. But when you’re constructing an ADU, you already own the land! That means every dollar you spend goes into the actual property.

Finding a builder

Finding a builder is one of the most important parts of the construction process—you get what you pay for.

It’s important to hire a professional who’s familiar with ADUs and with the ADU ordinances in your area. You’ll add months of time and headache by skimping on a good builder.

A quality builder will know how to design an ADU that will adhere to local ordinances, and he or she will also be able to accurately assess costs.

Before hiring any contractor or designer, ask questions and review samples of their past work.

Getting your ADU approved

You’ve found the perfect builder for your ADU. Now it’s time to start the building process.

First things first: make sure you and your builder understand the ADU regulations in your city. With that knowledge, you can start the permit approval process.

Each city has its own set of documents, but they’ll typically include things like an ADU Universal Checklist, ADU Building Plans Handout, an ADU Inspection Checklist, or something similar.

Fill out the necessary documents, and then submit your building plans to your local government (usually at City Hall). Make sure your builder or architect draws up your plans. A quality builder will know all the technical details to include.

Once your package is approved, you’ll start construction!

Constructing a detached ADU includes steps like establishing your foundation footings, installing underground utilities, adding structural rebar, concrete placement, framing, and more.

Key takeaways

Constructing an ADU isn’t easy and it isn’t cheap. However, ADUs can add massive value to your property and/or provide rental income that makes them worthwhile!

Before committing to an ADU, make sure you understand both who it’s for and why you’re building it. Your answers to those two questions will direct the rest of your planning and construction process.

When you’re ready to hire a contractor or a builder, remember: you get what you pay for!

Better to spend extra money on a quality, seasoned builder than waste time and money later dealing with someone who isn’t familiar with ADUs.

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