South Valley Spotlight - Backyard Housing
Estimate ADU build cost, permit fees, rent, tax impact, and payback years before you call a builder.
Use this if you own a Morgan Hill, Gilroy, or San Martin property and are trying to decide whether an ADU is worth the trouble. The first estimate needs to include construction, plans, permits, utility runs, rent, taxes, and the time it takes to get from idea to move-in.
The calculator below gives a planning range for detached units, attached units, garage conversions, and junior ADUs. It is built for local South Valley conditions, including city lots, county parcels, San Martin septic checks, and the cost difference between stock plans and custom plans.
A family ADU, a rental ADU, and a resale-value ADU are not the same project. A family unit may need privacy, accessibility, parking, and a layout that keeps everyone sane for years. A rental unit needs durable finishes, a clean entrance, and rent math that survives vacancy and repairs. A resale ADU has to make sense to the next buyer, not just the current owner.
Pick the use case first. Then the size, type, and finish level become easier to judge. A 499-square-foot junior ADU inside the existing house can be a very different financial move than an 850-square-foot detached cottage with new trenching, new utilities, and a custom roofline.
The output is a planning range, not a contractor bid. It estimates build cost, permit cost, likely rent, annual net rent after a vacancy and maintenance haircut, payback years, and the rough property-tax impact from new construction. The tax piece matters because an ADU does not usually reset the Prop 13 base on the existing house, but the new construction value can still add a separate assessed amount.
If the number looks close, move from calculator math to a site visit. The lot decides a lot. Utility distance, slope, access, trees, drainage, sewer, septic, and panel capacity can change a clean spreadsheet very quickly.
Use the South Valley real estate hub when the ADU is part of a buy, sell, or keep decision. Use the South Valley Directory when the next step is comparing local service pages.
This page is built around homeowners who are close enough to spend money. They are not reading a general design story. They are testing lot use, rent, taxes, family housing, and whether a builder call is worth making. That makes ADU traffic valuable even when total search volume is smaller than the home value or property tax pages.
For SVS, the goal is not to chase every ADU keyword in California. It is to own the South Valley version of the question, then connect those readers to calculators, permit explainers, real estate pages, and eventually vetted local service sponsors.