ADU 101

Let's start from the beginning

What is an ADU?

From space for your loved ones to building your retirement through renting, an ADU will grow with you and your family over time.

Accessory dwelling units (ADUs), also known as granny flats, casitas, in-law units, or backyard cottages, can come in many shapes and sizes and serve many different needs. ADUs are always a self-contained home that is often smaller than the main home and legally part of the same property. ADUs must have a kitchen, bathroom, and place to sleep, and can range from studios under 500 square feet to more than 1,000 square feet with multiple bedrooms.

ADU Types

Detached ADU

New freestanding structure separate from the main home, like a backyard cottage.

Pros

Cons

Project Timing

Building an ADU is an investment of time as well as money. Most projects take 12-24 months to complete, with interior conversions being the fastest.

Project Cost

Generally, the total cost to build an ADU in San Mateo County ranges from $450-$600 per square foot. The San Mateo County ADU Calculator is a great place to start when developing a budget. It provides a rough estimate of costs and income and will help you understand how choices can impact your budget over time, all customized to real local numbers.

Construction Types

Site-Built/Traditional

A traditionally constructed ADU, built from scratch on your property (“stick-built”). This option typically allows for a lot of customization and smaller changes made throughout the construction process.

Prefabricated / modular / manufactured

These ADUs are partially or mostly built in a factory, then shipped to your site to be put together. Sometimes the company will include all services in their fee (“turn-key”), including help with permitting and all on-site construction tasks (e.g., laying the foundation, utility hookups, etc.). Other times you’ll need to hire additional professionals to help.

Why build an ADU?

Building an ADU is an investment in your property, your family, and your community.

ADUs often provide homes for the local workforce and small families who have difficulty finding local housing that meets their needs. For homeowners, there are many reasons to build an ADU — rental income, safe and convenient housing for relatives, flexibility over time, and many others.

A Home As You Age

Space for a caregiver or an accessible and comfortable option for aging adults.

Housing loved ones

Homes for adult children, aging relatives, or family with special needs.

Environmental Benefits

Small-scale living and reduced commutes for local workers.

Rental income

Money for retirement, savings, or a rainy day from either the ADU or the main home.

Safety

An extra set of eyes on the property day-to-day or when you’re away.

Community support and workforce housing

Housing for neighbors and support for the local economy.

Frequently Asked Questions

Find answers to the most frequently asked questions about ADU basics.

Are you ready to ADU?

Explore this website and learn about the phases of ADU development.

Take the First Steps

Getting Started

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