Terms and Conditions

PLEASE READ THIS AGREEMENT CAREFULLY BEFORE USING THE CALCULATOR. BY USING THE CALCULATOR OR CLICKING “AGREE” YOU ARE AGREEING TO BE BOUND BY THIS AGREEMENT.  THE FOLLOWING TERMS APPLY. The calculator is an educational tool and not a substitute for professional advice. Users should not rely on any analysis or information generated from use of the calculator to make decisions about a particular project. The analysis and information generated from use of the calculator is understood to be generally accurate in San Mateo County as of the date the calculator was published/last updated, but every project is unique and User’s actual experience may be different. For instance, construction costs, monthly rents, insurance premiums, taxes and other information vary broadly based on local and personal circumstances. In addition, the calculator on this website does not factor in local zoning standards. User could use the calculator to estimate the cost of building a 1,100 square foot second unit, but applicable zoning regulations may cap the size of units at 850 square feet. Furthermore, the calculator and its underlying data are not updated regularly. Please see the assumptions page for more information about Home for All San Mateo County methodology and the date of the most recent update to the calculator and its underlying data. Home for All San Mateo County welcomes feedback on its methodology or how Users can make the calculator more useful. Please email Home for All San Mateo County at secondunits@smcgov.org Disclaimer. THE CALCULATOR, ANY INFORMATION OR ANALYSIS GENERATED THROUGH ITS USE, IS PROVIDED ON AN “AS IS” BASIS WITHOUT WARRANTIES OF ANY KIND, EITHER EXPRESS OR IMPLIED. THE COUNTY OF SAN MATEO AND ITS OFFICERS, AGENTS, EMPLOYEES, COMMUNITY PARTNERS AND SERVANTS DISCLAIM ALL OTHER WARRANTIES, EXPRESS OR IMPLIED, INCLUDING, WITHOUT LIMITATION, IMPLIED WARRANTIES OF MERCHANTABILITY, AND FITNESS FOR A PARTICULAR PURPOSE, AS TO THE CONTENT AND INFORMATION GENERATED BY USE OF THE CALCULATOR. Limit of Liability. The County of San Mateo and its officers, agents, employees, community partners and servants are not liable to User for any indirect, consequential, incidental or special damages (including without limitation, lost profits and lost data, information or content) arising out of this agreement, regardless of the theory of liability (including negligence and strict liability). Governing Law. This agreement is governed by the laws of the State of California (without regard to conflicts of law principles) for any dispute between the parties or relating in any way to the subject matter of this agreement. Any suit or legal proceeding must be exclusively brought in the federal or state courts for San Mateo County, California, and User submits to this personal jurisdiction and venue. Miscellaneous. This agreement constitutes the entire agreement between the parties and supersedes any prior or contemporaneous negotiations or agreements, whether oral or written, related to this subject matter.  User is not relying on any representation concerning this subject matter, oral or written, not included in this agreement. No representation, promise or inducement not included in this agreement is binding.  No modification or waiver of any term of this agreement is effective unless both parties sign it.

Second Unit Calculator

Beta Version

Use our calculator below to help estimate the costs, returns, and benefits associated with building an ADU.

Click here for information on how this calculator functions.
This calculator is an educational tool and not a substitute for professional advice.

Second Unit Basics

New construction includes both attached
and detached second units.

Conversion of part of house means
taking existing living space, e.g. a
basement or part of the house, and
turning it into a second unit.

Modular/panelized are built partially
or entirely off site and delivered
to the property.
If you change the number of bedrooms,
be sure to adjust the square footage accordingly
Required message
{{myForm.ADU_Sqft.$viewValue}} is below 150 sqft minimum
{{myForm.ADU_Sqft.$viewValue}} is above maximum sqft of {{ADU_Sqft_Max}} in {{Jurisdiction}}
Unit sizes vary significantly,
but typical square footage is:
Studio : 400 sqft

1 Bedroom : 600 sqft
2 Bedroom : 800 sqft
3 Bedroom : 1,200 sqft
Different levels of interior quality
(e.g. countertops, flooring, etc.) affect
the final construction price. Please choose
the interior quality that best matches your
planned construction.

Low cost: Rental quality. Entry level
appliances, low cost carpet, etc.

Standard: Typical for San Mateo County,
which means generally high quality. Stainless
steel appliances, solid countertops, a mix of
carpet and hardwood or tile.

High cost: Assumes the top end of anything
generally available at major retailers, ceramic
tile floors, etc. Costs would be higher if, for
example, you used a custom cabinet maker.
Required message
{{myForm.Construction_Cost.$viewValue}} is below $50,000 minimum
{{myForm.Construction_Cost.$viewValue}} is above $700,000 max
Individual site conditions can significantly
affect construction cost. The most common
reason construction is more expensive is
building on a hillside, but there are
many other potential challenges.

Rent and Financing

Required message
{{myForm.Loan_Term.$viewValue}} is below 1 year minimum
{{myForm.Loan_Term.$viewValue}} is above 30 year max
Typical loans are 20 years or less.?
Required message
{{myForm.Loan_Interest_Rate.$viewValue}} is above 7% max
Check with your local bank for today's rates?
Required message
${{myForm.Cash_Investment.$viewValue}} exceeds the ${{Cash_Investment_Max}} construction costs
Consider pricing your unit so a teacher or
nurse can afford it. You'll be helping the
community and they make great tenants!
Required message
{{myForm.Rent.$viewValue}} is below $700 minimum
{{myForm.Rent.$viewValue}} is above $7,115 max
Managing a second unit means finding a
tenant and doing repairs as necessary.
Typical costs to hire someone for
this are 5% of the rent.
There have not been enough second units
built to accurately estimate how much
it is likely to increase the value of
a house. The calculator assumes the
increase will be a multiple of the
rent. See assumptions below
Required message
{{myForm.Increased_Value.$viewValue}} is above 600,000 max

Advanced Features

Required message
{{myForm.Rent_Inflation.$viewValue}} is below 1% minimum
{{myForm.Rent_Inflation.$viewValue}} is above 10% max
Required message
{{myForm.Tax_Rate.$viewValue}} is below 1% minimum
{{myForm.Tax_Rate.$viewValue}} is above 1.5% max
Look up your tax rate at
Required message
{{myForm.Insurance_Rate.$viewValue}} is above 1% max
Required message
{{myForm.Years_Occupied.$viewValue}} is below 1 year minimum
{{myForm.Years_Occupied.$viewValue}} is above 120 year max
Required message
{{myForm.Weeks_to_Occupy.$viewValue}} is above 8 week max
Required message
{{myForm.Management_Rate.$viewValue}} must be above 0% minimum
{{myForm.Management_Rate.$viewValue}} is above 10% max

There is an unexpected number in
the information you've provided. Please review and update to see results.


{{Rent - Monthly_Costs[0] | currency : usd : 0}}

Monthly Income After Expenses

{{Rent | currency : usd : 0}} monthly rent and {{Monthly_Costs[0] | currency : usd : 0}} monthly costs
Hover over chart for monthly cost breakdown

Construction Costs

The cost to build your Second Unit is estimated at {{Total_Development_Cost/ADU_Sqft | currency : usd : 0}} per square foot with a total cost of {{Total_Development_Cost | currency : usd : 0}}.
Hover over chart for construction cost breakdown. Hard costs cover constructing the second unit, both labor and materials. Soft costs are things like architects. Fees are charged by the city.
{{Years_Investment_Recovered | number : 0}}

Years to earn back your cash investment

You will recover your initial cash investment of {{Cash_Investment | currency : usd : 0}} in {{Years_Investment_Recovered | number : 0}} years (excluding outstanding loan balance and increased value of house).
{{Years_ADU_Recovered | number : 0}}

Year Return on Investment

Once you account for the amount remaining on your loan and the increase in property value, your Second Unit will pay for itself in {{Years_ADU_Recovered | number : 0}} years.

Increase in Home Value

Your Second Unit will increase your home value by approximately {{Increased_Value | currency : usd : 0}}, assuming the value increases by 8-9 times the yearly rent.
Year 10
Year 20
Year 30
Property value increase
from second unit
{{Increased_Resale_Value[9] | currency : usd : 0}}
{{Increased_Resale_Value[19] | currency : usd : 0}}
{{Increased_Resale_Value[29] | currency : usd : 0}}
Rent collected (cumulatively)
{{Cumulative_Gross_Income[9] | currency : usd : 0}}
{{Cumulative_Gross_Income[19] | currency : usd : 0}}
{{Cumulative_Gross_Income[29] | currency : usd : 0}}
Cumulative net income
(rents minus costs)
{{Cumulative_Net_Income[9] | currency : usd : 0}}
{{Cumulative_Net_Income[19] | currency : usd : 0}}
{{Cumulative_Net_Income[29] | currency : usd : 0}}
Future monthly rents
{{Gross_Rental_Revenue[9]/12 | currency : usd : 0}}
{{Gross_Rental_Revenue[19]/12 | currency : usd : 0}}
{{Gross_Rental_Revenue[29]/12 | currency : usd : 0}}
Monthly costs
{{Monthly_Costs[9] | currency : usd : 0}}
{{Monthly_Costs[19] | currency : usd : 0}}
{{Monthly_Costs[29] | currency : usd : 0}}
Annual tax increase
{{-Property_Tax[9] | currency : usd : 0}}
{{-Property_Tax[19] | currency : usd : 0}}
{{-Property_Tax[29] | currency : usd : 0}}
Average annual return on investment (just using rent)
{{(Gross_Rental_Revenue[9]/12 - Monthly_Costs[9]) / Cash_Investment * 100 | number : 1}}%
{{(Gross_Rental_Revenue[19]/12 - Monthly_Costs[19]) / Cash_Investment * 100 | number : 1}}%
{{(Gross_Rental_Revenue[29]/12 - Monthly_Costs[29]) / Cash_Investment * 100 | number : 1}}%