Assumptions and Notes

The calculator is in Beta Testing. We appreciate your help if you notice any bugs or unrealistic assumptions please be in touch.

Construction Costs

New and garage conversion:

  • Garage conversions use the same cost assumptions as new construction because they often entail major work (e.g. replacing the roof; removing the slab concrete floor, adding a vapor barrier and re-pouring the concrete.) Sometimes this work is not necessary and the costs are lower.
  • $375 per square foot costs for the first 500 square feet. After 500 sf, the cost drops to $350 per sf.
  • Minimum cost of construction is $150,000. Multiply by 0.8 for low cost finishes and 1.2 for high cost finishes.
  • $15,000 for each bathroom more than typical (e.g. 1 bedroom, 2 bath would be $15,000 more).
  • Use a multiplier of 1.2 – 1.5 for construction costs if the conditions/construction is difficult.

Conversion of part of house:

  • $40 per sf for finishing, sanding, etc.
  • $22,500 for a new bathroom
  • $30,000 for a new kitchen ($10,000 for a partial kitchen),
  • $10,000 each if you are separating or installing new water, electrical or heating system.
  • Later versions of the calculator will differentiate between finishing an attic or basement and splitting the house in two.


  • Costs of $350 per sf, dropping to $300 per sf if over 500 square feet.
  • Minimum cost of construction is $100,000 for a modular unit.
  • Same multipliers for difficult conditions or high end finishes.

Soft Costs: Soft costs calculated at 35% of the calculations described above. An architect is an example of a soft cost.

Fees: Fees vary based on recent projects in the jurisdiction. The calculator uses county averages for some or all of the fee data for the following cities โ€” Atherton, Burlingame, Daly City, East Palo Alto, Foster City, Menlo Park, Millbrae and Woodside. Fees are in addition to the costs above.

Source: Developer interviews, review of permits.


5% of rent

Source: Management company interviews

Property Value Increase

  • 100 times the monthly market rate rent for new construction/modular.
  • No increase if it is a conversion of part of a house. In a future version of the calculator, a separate calculation will be provided for converting a basement, which will increase value from splitting a house in two as compared to converting part of a house, which will not increase value.
  • The value for converting a garage is 75 times monthly rent.

Source: Realtor interviews


0.25% of construction cost. Year 1 construction costs based on calculations as detailed above. Construction costs are then increased over time by 5% a year, meaning the insurance costs will be higher in year 10, even though they are still based on the formula, 0.25% times construction cost.

Source: Insurance companies.


2% per year.

Vacancy Rate

4% (2 year average length of tenant, 2 weeks between rentals)

Average Rent Increase

  • Teachers โ€“ 2% per year.
  • Others โ€“ 5% per year

Property Value Increase

5% per year

Source: Based on historical data, but the number varies dramatically depending on what time period you are looking at.

Repairs and Capital Replacement Costs

0.5% of construction costs, reduced for years 1-10.

Source: Conversation with builders and architects

Tax Rate


Source: Assessor and Comptroller data.

© San Mateo County. All rights reserved.
Photo credit: Home page hero by Josh To,; home page Getting Started section photo by New Avenue Homes.

Translate ยป