Accessory Dwelling Units (ADUs)

Photo Credit: Kelley Kelso

Commonly referred to as secondary dwelling, mother-in-law, or granny units, ADUs are smaller buildings that share the same lot of a larger, primary residence. Examples include a detached guesthouse or a garage with a rented apartment above. ADUs have many benefits: they are an affordable type of home to construct because they are built on land that is already owned, they provide an income source to homeowners, they are typically less costly to construct than new multifamily developments, and provide a flexible living option for individuals, family members, couples, or seniors. Because of these benefits, ADUs have been identified in jurisdictions around the country as one solution to increasing the supply of workforce and affordable housing. Examples of ADUs include a detached guest house or a garage with a rented apartment above.

With the growing amount of research around ADUs and benefits they provide to communities, TRPA is in the process of considering policies that would do more to encourage ADUs as an affordable housing type. Please see the “Phase 1 Housing  Amendments Approved” section in the FAQ below to learn about recommendations recently approved by the TRPA Governing Board.

Based on these recommendations, as of September 29, 2021, the TRPA Code allows ADUs on all California residential parcels, subject to local jurisdiction rules and regulations. In Nevada, ADUs are only allowed on parcels larger than one acre.

Frequently Asked Questions

Phase 1 Housing Amendments Approved

The Tahoe Regional Planning Agency (TRPA) Board adopted updated rules to encourage more affordable housing options for Tahoe residents and workers at its July 29, 2021 meeting. The TRPA Code amendments are an initial outcome of the housing initiative the agency launched last year and were developed with consultation from the multi-stakeholder Tahoe Living Housing and Community Revitalization Working Group, a committee of the Advisory Planning Commission. With regards to ADUs, the amendments:

  • Allow ADUs on all California residential parcels, subject to local jurisdiction rules and regulations. The new rules do not change the 1-acre or larger restriction for ADUs in Washoe and Douglas Counties, which is consistent with current county rules.
  • Allow up to two ADUs on all California residential parcels, to accommodate one detached ADU and one attached ADU.
  • Remove requirements for noticing to adjacent neighbors, similar to rules for other accessory single-family uses.
  • Require ADUs to still comply with existing coverage regulations.
  • Require ADUs to still comply with existing development rights requirements.

The new rules take effect on September 29, 2021.

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How do I begin the process to build an ADU?

All ADU applications are currently processed first through TRPA, then through your local jurisdiction for a building permit. For the TRPA application, ​use the Single-Family Dwelling Application. Once your application has been received, it will go through the TRPA permitting process. If you have additional questions about your project or the application process, you can call the front counter at 775-588-4547, or for extensive questions, set up a pre-application meeting.

For more information on local jurisdiction ADU requirements visit:

What is the difference between accessory living space and an ADU?

An ADU is different from an accessory living space because an ADU is a standalone living space with a kitchen. Per TRPA Code Chapter 21, examples of residential accessory living spaces include garages, green houses, and art studios, to name a few. Accessory living spaces do not require a development right or a bonus unit, whereas ADUs do.

How many ADUs can I build on my property and what size can they be?

In California, TRPA allows up to two ADUs on a single-family property. Local jurisdictions’ building codes in California (Placer County, El Dorado County, and the City of South Lake Tahoe) further specify the number and size of ADUs.

City of South Lake Tahoe:
Information on building an ADU within the City of South Lake Tahoe can be found here: https://www.cityofslt.us/1191/Accessory-Dwelling-Unit-FAQs

Placer County:
Information on building an ADU within the Tahoe Basin portion of Placer County can be found here: https://www.placer.ca.gov/DocumentCenter/View/55810/Assessory-Dwelling-Units-in-Placer-County-Tahoe?bidId=

El Dorado County:
Information on building an ADU within the Tahoe Basin portion of El Dorado County can be found here: https://www.edcgov.us/Government/building/pages/residential_second_dwellings.aspx

What are development rights and which development rights are required to build and ADU or Junior ADU?

Development rights are assets that are tied to a property and are rights that someone must acquire before a new property is developed. Development rights include tourist accommodation units (TAUs), single and multi-family residential units of use (RUUs), commercial floor area (CFA), and coverage. Residential units of Use (RUUs) are formed by combining a potential residential unit of use (PRU) and a residential allocation, or by converting a TAU or CFA.  

A Residential Unit of Use (RUU) is required to build an ADU that is not income-restricted in the Tahoe Basin. Unused or underutilized development rights can be bought on the open market and used for projects, including building an ADU. More information on development rights and how to obtain them can be found here. As of January 2022, the price for a residential development right on the market, or for a TAU or CFA that can be converted to a residential development right, ranges from around $3,000 to $30,000, depending on type, availability and jurisdiction. 

Is there a way to build an ADU without acquiring a development right?

If your property is within ½ mile of an existing transit stop, ½ mile from a town center, or within an area zoned for multi-family housing, you are eligible to receive a development right for your ADU at no cost (this development right is called a “bonus unit”). TRPA awards bonus units in exchange for deed-restricting a unit to affordable, moderate, or achievable income levels. Deed restricting means a property owner has officially recorded through the assessor’s office that the property must be used in accordance with the income restrictions.

A bonus unit takes the place of a Residential Unit of Use. Receiving a bonus unit at no cost can help many property owners create an ADU they would not have been able to otherwise. By awarding bonus units in areas close to transit and town centers, the creation of new ADUs aligns with Lake Tahoe Regional Plan goals to make Tahoe communities more walkable, bikeable, and reduce reliance on cars.

For more information on bonus units and deed restriction requirements, please visit the TRPA’s Housing website and the  Residential Bonus Unit Factsheet.

Am I eligible for a bonus unit and how do I start the process to receive one?

You can check the map here to see if your property is within the Residential Bonus Unit Boundary and thus eligible for a bonus unit. If you would like to utilize a bonus unit for your ADU, indicate that in your application and you will be awarded a bonus unit during the permitting process.

My property already has the maximum allowable coverage, can I build an ADU?

In order to build a new, detached ADU, you must have the necessary amount of coverage available to accommodate an ADU on your property. To determine how much coverage exists on your property and how much could be added, apply for a site assessment. Note that over-the-garage ADUs and re-purposing existing space that is already using legally verified coverage do not require additional coverage.

Do I need to complete my Best Management Practices (BMPs) to build an ADU?

Yes. All Best Management Practices (BMPs) are required on both the primary home and the ADU. All stormwater must be infiltrated on site, and temporary BMPs must be installed during construction.

Calculate Costs

Use the ADU Calculator Tool to estimate your overall costs and return on investment for building an ADU in the Tahoe region. Note, this tool should be used for rough estimates only and costs may vary by project and as construction costs fluctuate. Not all jurisdictions have provided cost estimates. The tool was last updated in May 2022. 

TRPA Fees and other Associated Cost Requirements

Some TRPA fees or cost requirements are waived for projects that are deed-restricted affordable, moderate, or achievable. See the following list of fees and other associated costs for ADUs. Fees and costs with an asterisk* are waived for deed-restricted ADUs.

Fees and Costs effective January – December 2022:

*Application fee: $1.45 per sq. ft of floor area covered by roof ($727 minimum) + $140 BMP Fee +  $127 IT Surcharge
*Mobility Mitigation Fee – Depends on location, ranges between approximately $3,000 – $10,000.
Water Quality Mitigation Fee – $1.82 per square foot of base allowable coverage.
Excess Coverage Mitigation Fee and/or Coverage Transfer purchase price if applicant is building more coverage than the base allowable – Fees vary depending on hydrologic area.
Security – Varies between $2,000 – $3,000 depending on size of project.
Security Administration Fee – $223 (Refundable).
*Residential Development Right – varies, consult with local jurisdiction or see Development Right Fact Sheet.

For the most up-to-date information on fees see the Fee Schedule section of the Applications and Forms page.