How to Build an ADU?

Building your ADU project from start to finish.

When deciding to build an ADU project, the first questions you may ask yourself are: How does it work? What does the process look like and who do I contact first? We’ll dig into the professional relationships and project structures you’ll need to familiarize yourself with in order to make the project a success.

Architect + Contractor

The most common way for homeowners to design, permit and build their ADU is by working with an architect and contractor who separately manage their project through each step from initial design to final inspection and occupancy.

The benefits of this model are that you have specialized professionals who are highly capable of managing the two critical phases of any construction project; Design and permitting (architect) and then building and inspections (contractor).

The downsides are that no one entity has full responsibility for the success of your project (other than yourself) and also that sometimes contractors and architects have misaligned interests. For example, an architect may want the design to be an award winning piece of architecture and may not fully understand the costs of various features and characteristics in their designs from a buildability perspective. Efficiency and buildability are typically what matters most to contractors.

For this reason, it is critical that you establish a productive working relationship between the architect and contractor before committing to move forward. It may also be helpful to choose architects and contractors who have worked together on previous ADU projects and completed them successfully.

This is a good model for those who would like a very unique design and are willing to spend a bit more to get it.


Design and build contractors are a great fit for homeowners who are looking to find a one stop shop for their ADU project. These contractors specialize in providing all project services in one package including feasibility, design, permitting, and construction. This makes it easy for a homeowner to complete their project with a single point of contact. Design/build contractors have a tendency to be more efficient for conversion and new construction ADU projects because they are operating with a tighter iterative design process and they are typically specialized for these kinds of projects. Design/build, generally speaking, is a good option if you are interested in a simple, efficient process that is within a predictable budget.

Prefab/Manufactured Units

While still only a small fraction of the total construction projects annually, prefabricated and manufactured modular units have been showing promise in recent years. Modular units can present an attractive option for building a Detached ADU in many areas. It is important to consider a variety of issues when researching whether modular is the right solution for you.

  1. Determine your need for customization: In many neighborhoods, design restrictions require the ADU to be built with the same exterior materials and characteristics as the primary home. This is a hang up for many modular manufacturers who have standardized options and feature selections.
  2. Research the building code required in your local area. Many modular units are built to the HUD code, which is a lesser national building code and is not allowed in nearly all zoned residential areas. In California for example, all units must be built and inspected to the California Building Code 2016 and meet all requirements for fire safety and energy conservation. If the model is standardized and pre approved, make sure that its design will be compliant with your local codes using a model spec sheet from the manufacturer.
  3. Cost expectations and time; It is a common misconception that modular units are always less expensive and faster than on-site construction. While this is true in some cases, it should not be taken for granted.
  4. Who is handling what? - It is also important to understand the scope of work for a modular manufacturer. In almost all cases, you will still need to work with an architect to obtain the necessary permits and a local contractor to prepare the site, install the foundation in set the unit. Be sure to ask the manufacturer, have they already installed a unit your area? If so, do they have contractors that you can work with to estimate the steps of the project which are outside their scope?
  5. All in all, while there is still much education and development needed to further modular and prefabricated units as a quick and easy solution for ADUs, they have also come a long way over the last decade and they present an interesting alternative to on-site construction for the future of this movement.


    The first instinct for many homeowners is to take a Do It Yourself approach. This is great, but truly ask yourself, do you have the time and experience to manage each step of the project? What are the reasons for DIY? - To save money, or speed up the process? If this is the case, beware that this is a misconception among many homeowners who get started on projects, only to find that they are missing critical steps, or the amount of time required to learn and then perform each step causes the project to cost far more than if they had hired professional services from the beginning.

    One possibility is to plan to handle part of the process, but to still enlist the services of professionals. For example, if you are inclined to design, drafting, and paperwork, maybe managing the permitting process with some supplemental consultation on the local regulations is the right amount of DIY for you.

    DIY is a great approach if you feel confident that you know what you are getting into, or at the very least, you are willing to accept many setbacks and delays along the way. If not, we’d highly suggest working with professionals on your project.

    Project Financing

    Regardless of the model that you choose to go with, you will need some source of financing to move forward with your project. This is always better to get sorted out as a first step, or at the very least, early on in the design and feasibility process. Getting qualified for financing will help you determine what your budget is for the project and what the returns will be (if you intend the ADU to generate income). Reaching out to lenders as a first step, will also give you more concrete terms to discuss with architects and contractors as you move into the estimating phase of your project.

    Read through more resources for ADU financing, designs and regulations in our knowledge base and learn more about your local requirements by finding your City Page here.