Estimate Your Costs
Once you have an idea for how you will add an ADU to your property, these tools will help you estimate your costs. Minimize the Cost of Your ADU.
Start with the Fee Estimator to get an estimate of the permit and impact fees that will apply to your project – fees vary based on type of construction and number of bedrooms. You will need to enter your Assessor’s Parcel Number in the Fee Estimator.
Find your APN here:
Estimate your costs here:
With a fee estimate, and construction cost estimates, you can download the Construction Cost and Cash Flow Estimator to collect all your costs in one place and start determining how you will finance the project. If you intend to rent your ADU the Cash Flow Estimator will help you see how different loan terms and monthly rents will affect your monthly cash flow.
Sample completed spreadsheets for the estimator tool are found here: Construction Cost and Cash Flow.
The Financing Guide has extensive information that will help you consider all the costs associated with creating an ADU, as well as providing an overview of some of the responsibilities related to owning a rental property.
Minimize the Cost of Your ADU
There are many styles of ADUs, beyond the cute, stand-alone "back yard cottage," which is the most expensive type of construction. Consider all the options that could meet your housing needs before settling on a style of construction and location for your ADU.
Consider whether a remodel to your existing single family home would meet your needs.
Remodels can be substantially cheaper than construction of a full, independent dwelling unit. Remodeling to create greater separation between living spaces while not creating a separate dwelling unit can accommodate the needs of many households.
A second area for limited food prep that does not include a full stove or oven is allowed in addition to the kitchen in an existing home. Essentially, a sink, refrigerator, counter space, and any small appliances that plug into a standard 110V wall outlet can be added to a bedroom or den to create a separate living space in the main home. The separate space must maintain an interior connection to the rest of the home, since it is not a fully independent ADU.
Consider a Conversion ADU
If there is existing habitable space in your home or an outbuilding that is suitable to be converted to an ADU, both construction and permitting costs are substantially reduced. In order to be a full ADU, the finished space must have a full bathroom, full kitchen, separate entrance, separate heat source, and sound and fire-wall separation from the primary home.
Consider converting an existing garage
Do you have space to relocate your existing required parking space(s) outside the garage and use the garage as an ADU? Parking spaces can be replaced with uncovered parking in tandem or triple tandem (2 or 3 cars parked behind one another) to accommodate an ADU on your lot. See parking requirements below.
You may want to consult with a contractor regarding the condition and quality of the existing structure to see if converting is financially more feasible than new construction.
Consider an addition to an existing structure
Additions to existing homes, depending on the age of the existing structure, can also be a more cost effective approach to creating an ADU, especially for smaller units, than new detached construction.
Additions under 500 sf are exempted from certain permit reviews and fees (such as school fees, affordable housing impact fee and soils report review, etc. ), creating the potential for considerable cost savings.
Consider pre-fab homes
Example pre-fab Accessory Dwelling Unit
If detached new construction is the style of ADU that works best for your needs, consider whether a manufactured home or a prefab home would work on your site. Factory-built housing is legal to use as an ADU so long as it is attached to all required utilities and permanently mounted to an appropriate foundation. These units can represent significant construction cost savings in many locations, and require fewer building inspections for a quicker timeline to occupancy.
See a list of Resources for links to manufactured and pre-fab home dealers.
Explore various site-planning techniques
Once you’ve settled on a style of construction, consider site planning to minimize costs:
If you use a septic system, consider its location and elevation relative to a new ADU, and what additional costs this could create. Consult with County Environmental Health Services (scceh.com or 454-2022 for septic evaluation) early in your process.
ADUs that are built as free-standing New Construction trigger a requirement of 1 parking space, which must be accommodated on site.
In order to add any type of ADU, the main home on the property must meet current requirements for parking spaces in a garage, carport, or driveway; OR If the existing home does not meet current parking requirements, and the ADU does not trigger a requirement for a new parking space, the existing number of parking spaces must be maintained. As long as there is no net loss of off-street parking spaces on the property, spaces can be relocated and reconfigured in any manner that will fit on the parcel, including in tandem or triple tandem (three cars parked behind one another). There is no requirement that any of the parking spaces be covered by a carport or garage.
|Single Family Home Parking Standards
||2 parking spaces
||3 parking spaces
||3 parking spaces
||3 parking spaces
ADUs created as Conversions, or which are attached to the existing home or garage do not trigger a parking requirement, and therefore also do not trigger transportation-related fees.
On parcels of 6,000 sf or smaller, the County allows a 2% bonus above the standard lot coverage and floor area ratio limits in order to accommodate an ADU.