If you have been thinking about adding an accessory dwelling unit or expanding your home, there has never been a better time on the Central Coast. California law now requires cities and counties to approve most ADU applications ministerially — no public hearings, no discretionary review — and housing demand across the Monterey Peninsula and Salinas Valley makes a well-built ADU one of the strongest investments a homeowner can make.
RC Builders Inc is a licensed general contractor (CSLB License #1081425) headquartered in Seaside, and home addition and ADU projects are the core of what we do. Because we self-perform the trades that matter most on an addition — foundation and concrete work, framing, roofing, and finish carpentry — one accountable crew takes your project from the first site visit to the final inspection.
We build ADUs and additions in Seaside, Monterey, Salinas, Marina, Pacific Grove, Carmel-by-the-Sea, Pebble Beach, Carmel Valley, and throughout Monterey County.
What we build
Every property and every family is different, and state ADU law gives you more options than most homeowners realize:
- Detached ADUs — a standalone backyard home, typically 400 to 1,200 square feet, with its own kitchen, bath, and entrance.
- Garage conversions — often the fastest, most economical path to a legal rental unit or in-law suite, because the foundation, walls, and roof already exist.
- Attached ADUs — an addition to the main house with a separate entrance, sharing one or more walls.
- Junior ADUs (JADUs) — up to 500 square feet carved out of the existing home, with an efficiency kitchen.
- Room additions — primary bedroom suites, family rooms, home offices, and expanded kitchens.
- Second-story additions — the answer when the lot is small but the household is growing, common on Peninsula lots.
Monterey County ADU rules, in plain English
ADU permitting is where most projects stall, and it is where a local contractor earns their keep. The short version of current California law: your city or county must approve or deny a complete ADU application within 60 days, detached ADUs of at least 800 square feet generally cannot be denied on lot-coverage grounds, side and rear setbacks are capped at four feet for most new detached units, and when you convert a garage the city cannot require you to replace the parking. Development impact fees are waived entirely for ADUs under 750 square feet.
The details still vary by jurisdiction — Seaside, Monterey, Salinas, Marina, and unincorporated Monterey County each run their own permit counter, and properties in the coastal zone can involve additional review. We prepare and shepherd the full application, coordinate the plans, and handle every inspection, so you never have to stand in line at a planning counter.
What an ADU costs on the Central Coast
Honest numbers, because that is the first question every homeowner asks: on the Monterey Peninsula most garage conversions land between roughly $130,000 and $220,000, attached ADUs between $250,000 and $400,000, and new detached ADUs between $300,000 and $500,000 or more depending on size, site conditions, and finishes. On a per-square-foot basis, plan on roughly $400–$650 for new detached construction in this market.
The biggest cost drivers are utility connections, foundation and site work on sloped or sandy lots, and the finish level you choose. We give you a line-item estimate up front — not a per-square-foot guess — so you can make decisions with real numbers.
Room and second-story additions
Not every project needs a separate dwelling. A well-designed addition — a primary suite, a larger kitchen, a second story — often solves the space problem while adding more value per dollar than moving would cost. The craft is in the tie-in: matching the existing foundation, roofline, siding, and interior finishes so the addition reads as original to the house. That is finish-carpentry and framing work we do with our own crew, not a rotating cast of subs.
Why homeowners choose RC Builders for additions
Additions and ADUs touch every trade — concrete, framing, roofing, plumbing, electrical, insulation, drywall, tile, paint. Hiring a contractor who self-performs the structural and finish trades means fewer handoffs, a tighter schedule, and one phone number when you have a question. We are licensed, insured, local to Seaside, and we serve clients in both English and Spanish.
Our work
Home Additions & ADUs services we offer
- ADU construction
- Garage conversions
- Junior ADUs (JADUs)
- Room additions
- Second-story additions
- Primary suite additions
- ADU design & permitting
- Site work & utility connections
Frequently Asked Questions
Most detached ADUs on the Monterey Peninsula cost between $300,000 and $500,000 built, or roughly $400–$650 per square foot. Garage conversions are the most economical option, typically $130,000–$220,000, because the structure already exists. Utility connections, site conditions, and finish level drive the spread — we provide a line-item estimate after a site visit.
California law requires your city or county to approve or deny a complete ADU application within 60 days. In practice, plan on two to four months from starting design to permit in hand, depending on the jurisdiction and how quickly plan-check comments are resolved. We prepare and manage the entire application.
State law lets you build a detached ADU of at least 800 square feet on virtually any single-family lot, and most local ordinances in Monterey County allow up to 1,000–1,200 square feet depending on bedrooms and lot size. Junior ADUs are capped at 500 square feet within the existing home. We confirm the exact envelope for your parcel before design begins.
Yes. Long-term rentals (30 days or more) are allowed, and ADUs are one of the strongest rental investments on the Peninsula given local housing demand. Short-term vacation rental rules are much stricter and vary by city, so do not count on nightly-rental income without checking your jurisdiction first.
For standard ADUs, state law currently prohibits cities from imposing owner-occupancy requirements on units permitted after 2020. Junior ADUs are the exception — a JADU does require the owner to live in one of the units. Rules evolve, so we confirm current requirements for your jurisdiction during design.
No. State ADU law prohibits cities from requiring replacement parking when a garage is converted to an ADU. Losing covered parking is still worth weighing — but it is your call, not the city’s.
Once permits are issued, a garage conversion typically takes 3–4 months of construction, and a new detached ADU 5–8 months depending on size and site work. End to end — design, permitting, and construction — most projects run 8–12 months.
No. Your existing home is not reassessed. The county adds the assessed value of the new construction to your existing assessment, so you pay additional tax only on the value the ADU adds — typically a small fraction of the rental income a unit generates.