ADU & Home Additions in San Mateo
Expand your San Mateo home with a custom ADU or addition — designed for family, rental income, or simply more space to live.
How Much Does ADU & Home Additions Cost in San Mateo?
Detached ADUs in San Mateo. Garage conversions from $95K-$175K. Second-story additions vary by scope.
Every project is unique. We provide a fixed-price contract after your free consultation — no hidden fees, no cost-plus surprises. The price you agree to is the price you pay.
What to Know About ADU & Home Additions in San Mateo
Local building codes, neighborhood character, and permit requirements vary by city. Here's what San Mateo homeowners should know before starting a project.
- 01San Mateo processes ADU permits through its Community Development Department, generally adhering to California's 60-day review timeline; well-prepared applications with complete engineering are approved faster
- 02Hillsdale and Hayward Park lots (typically 5,500-7,500 sq ft) can accommodate detached ADUs up to 1,200 sq ft under state law, with 4-foot rear and side setbacks
- 03San Mateo participates in San Mateo County's pre-approved ADU plan program — selecting a pre-approved design can reduce permit review time by 2-4 weeks
- 04Second-story additions in San Mateo require structural engineering to verify the existing foundation can support additional weight; foundation reinforcement adds $15,000-$40,000 if needed
- 05San Mateo's utility connection fees for new ADUs include water, sewer, and electrical; budget $12,000-$25,000 for utility work depending on distance from existing services
Our ADU & Home Additions Process
A clear, structured approach from your first call to final reveal.
Feasibility Assessment
We evaluate your property, zoning, and lot coverage to determine what's possible and what will deliver the best ROI.
Design & Engineering
Our architects create plans that maximize space, light, and livability while meeting all local building codes.
Permits & Approvals
We navigate the full permitting process — city planning, building department, and utility coordination.
Construction
Our experienced crew builds your ADU with the same quality standards as our luxury home remodels.
Inspection & Handover
We manage all inspections, obtain the certificate of occupancy, and walk you through your completed ADU.
ADU & Home Additions in San Mateo: FAQ
Answers to common questions from San Mateo homeowners.
California state law allows detached ADUs up to 1,200 sq ft on most San Mateo lots, regardless of lot size or existing lot coverage. You can also build a Junior ADU (JrADU) up to 500 sq ft within your existing home in addition to the detached unit. These are statewide minimums that override local zoning restrictions.
The full timeline from initial design to move-in is typically 10-14 months. This includes 2-3 months for design and engineering, 2-3 months for permitting, and 5-8 months for construction. Garage conversions are faster — typically 8-10 months total. Barcci Builders provides a detailed schedule at contract signing.
Yes — second-story additions are one of our most popular San Mateo projects, especially in Hillsdale where single-story ranches dominate. We add primary suites, bedrooms, and bonus rooms above existing first-floor footprints. Structural engineering ensures the existing foundation can carry the load, and we match exterior materials for a seamless look.
A 1-bedroom ADU in San Mateo currently rents for approximately $2,400-$3,500/month depending on size, finishes, and location. A 2-bedroom unit can command $3,000-$4,500/month. This rental income can substantially offset mortgage payments and provides strong return on investment over time.
Yes. A well-built ADU in San Mateo typically adds $200,000-$400,000 in property value — often exceeding construction costs. The combination of added square footage, rental income potential, and flexibility (home office, guest house, multigenerational living) makes ADUs one of the strongest property investments available.
In most cases, no. California law eliminates parking requirements for ADUs located within half a mile of public transit — which covers much of San Mateo, including areas near Caltrain and SamTrans routes. Even garage conversions don't require replacement parking. This makes ADU construction simpler and less expensive.
More Services in San Mateo
Explore our other design-build services available in San Mateo.
Ready to Start Your San Mateo ADU & Home Additions?
Book a free in-home consultation. We'll discuss your vision, walk through your space, and provide a detailed fixed-price estimate — no pressure, no obligation.