Terms and Conditions


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}}%