A free site visit, a check of what your lot allows and a written, line-item price.
Get your free estimateADU Company in Berkeley, CA
Benson ADU Builders is an ADU company serving Berkeley, CA, with written pricing, real-time project tracking, and a 365-Day Done Right Promise on every project. Berkeley ADU work is rarely a simple “drop a box in the backyard” job. The housing stock is old, the lots are tight, and the city’s ADU ordinance is famously generous but exacting. That combination means most projects here involve seismic work, foundation fixes, or garage-to-ADU rebuilds rather than a clean slab pour. Call (424) 328-5746 for a free estimate, and we’ll walk you through what your specific property actually needs.
Why Berkeley Homes Are Different
Berkeley’s housing stock is dominated by early-20th-century Craftsman bungalows, brown-shingle houses, and Victorian-era flats. Many have unreinforced masonry or cripple-wall foundations that require seismic upgrades before or during ADU construction. We’re not being dramatic – it’s genuinely the case that most pre-1950 Berkeley foundations were never designed for the lateral loads the city now requires, so any addition that ties into the main structure means bringing that connection up to code.
In the flats, ADUs are overwhelmingly carved out of pre-1940s wood-framed homes and garages. Berkeley’s pioneering but exacting ADU ordinance allows up to 1,200 sq ft on any single-family lot – generous by Bay Area standards – but seismic retrofitting and soft-story garage conversions are often the real project drivers. In neighborhoods like South Berkeley, Elmwood, and the Westbrae area, we regularly see garages with discontinuous foundations and floor slabs that slope toward the alley. Berkeley’s mild marine climate also means interior ADUs rarely need air conditioning, but those uninsulated 1920s garages and basements almost always need serious insulation and moisture control to meet current energy codes.
Our Main Berkeley Services
The three services Berkeley homeowners call us about most are ADU construction, home additions, and room additions – each has its own page with full pricing and local detail below.
ADU Builders in Berkeley
Berkeley’s ADU ordinance makes accessory dwelling units an attractive option, but the build rarely starts with a clean site. A garage conversion in the flats almost always means dealing with a discontinuous foundation and a floor slab that slopes toward the alley – so much so that many Berkeley homeowners choose a full garage-to-ADU rebuild rather than a retrofit. We build with prefab brands like Abodu, Dvele, and Plant Prefab when the site allows panelized delivery, and we handle the Berkeley-specific engineering for seismic ties, soft-story bracing, and foundation work. Every project gets a written price before anything starts. See the full breakdown on our ADU Builders in Berkeley page.
Home Addition Contractor in Berkeley
Adding square footage to a Berkeley home means matching a structure that might be a century old. That’s not just an aesthetic question – it affects foundation load, seismic connections, roofline transitions, and energy compliance. We’ve worked on brown-shingle additions in the Berkeley Hills and bungalow expansions in the flats, and the common thread is that the existing foundation usually needs work before the addition goes up. We coordinate structural engineering, pull permits, and give you a fixed written price upfront. Learn more on our Home Addition Contractor in Berkeley page.
Room Addition Contractor in Berkeley
A room addition in Berkeley often means extending a single-story Craftsman outward or converting an underused porch or storage area. These jobs are smaller than full additions but they hit the same issues: foundation connection, seismic requirements, and matching roof and siding so the new space doesn’t look bolted on. We build with James Hardie cladding and other materials that hold up to Berkeley’s damp winters and dry summers. Pricing is always written and fixed before we begin. Details are on our Room Addition Contractor in Berkeley page.
What happens when you call
- 1
A real person answersNo phone trees - you reach a local pro.
- 2
You get an upfront price rangeHonest numbers before anyone is dispatched.
- 3
A background-checked tech heads outLicensed & insured, dispatched right away.
- 4
You approve before work beginsNothing starts until you say go.
Brands We Service in Berkeley
We build with and service prefab systems from Boxabl, Plant Prefab, and Tuff Shed, plus James Hardie cladding and trim, across Berkeley. Panelized kits usually ship within 4 to 8 weeks once plans are approved – we handle delivery, craning, and installation with our own background-checked crews. See our brands for the full product lineup.
Pricing for ADU Company in Berkeley, CA
| Service | Typical range in Berkeley |
|---|---|
| Detached ADU (site-built, 1-bed) | $180,000 – $275,000 |
| Detached ADU (prefab panelized, 1-bed) | $150,000 – $240,000 |
| Garage-to-ADU conversion (full rebuild) | $140,000 – $220,000 |
| Garage-to-ADU retrofit (existing shell) | $90,000 – $160,000 |
| JADU (attached junior unit) | $80,000 – $130,000 |
| Home addition (400 – 800 sq ft) | $165,000 – $310,000 |
| Room addition (150 – 350 sq ft) | $75,000 – $140,000 |
| Second-story addition (400 – 700 sq ft) | $220,000 – $420,000 |
| Seismic retrofit + foundation replacement (standalone) | $35,000 – $90,000 |
| Full plans + engineering + permit package | $12,000 – $28,000 |
The biggest price drivers in Berkeley are foundation condition and seismic requirements. A garage in the flats with a broken slab and no rebar will cost more to fix than a level, intact one. A hillside addition on a downslope lot pushes engineering costs up significantly. Whatever the range, you get a written price before we start – and it doesn’t change at the door.
Neighborhoods We Serve in Berkeley
Our crews work throughout Berkeley, from the flatlands to the hills. We schedule projects in this area Monday through Saturday, with most estimates happening within a few days of your call.
- South Berkeley
- Elmwood
- Westbrae
- North Berkeley
- Berkeley Hills
Service Area – Cities Near Berkeley
We serve Berkeley and the surrounding East Bay communities. If you’re just outside the city line, feel free to call – we also work in Albany, CA, Piedmont, CA, Pinole, CA, and across the Bay in San Francisco, CA.
Serving Berkeley, CA - Our Local Coverage Area
We’re based in the Berkeley area and know this community well. Use the map below to see our service coverage - if you’re nearby, we can almost certainly help.
FAQs About ADU Company in Berkeley
Site-built ADUs in Berkeley typically take 9 to 14 months from first call to move-in. Prefab panelized units can cut that to 6 to 10 months. The biggest variable is Berkeley’s planning and building department queue. Call (424) 328-5746 and we’ll give you a realistic timeline for your property.
Yes. Every project includes full plans, engineering, and permitting, with the permit package priced into your written quote. Berkeley’s ADU review process requires specific seismic and zoning documentation, and we’ve worked with the city’s requirements on garage conversions, setbacks, and soft-story retrofits.
No. You receive a written line-item price before any work begins, and it’s fixed unless you request a change scope change. Change orders are priced in writing and signed by you before any new work proceeds. That’s the Relay Visibility Guarantee, and it’s stamped on every contract we issue.
Get Your Berkeley ADU Estimate
If you’re adding an ADU, home addition, or room addition in Berkeley, call (424) 328-5746 or reach out through our home page. We’ll schedule a free estimate, walk the site with you, and send a written quote within days – no pressure, no vague pricing, just numbers you can plan around.
What happens when you call
- 1A real person answersNo phone trees - talk to a local adu pro.
- 2You get an upfront price rangeHonest numbers before anyone is dispatched. No surprises.
- 3We come out and measureLicensed & insured, at a time that suits you - the visit and the quote are both free.
- 4You get the price in writingLine-itemed, yours to keep, and nothing starts until you say go.
What Berkeley Customers Say
"Garage conversion into a guest suite. I could see the permit status in the app, which made it painless."
- Rick F."Solid job. Cabinets arrived late but they adjusted the schedule."
- Eli K."Plumbing and electrical passed on the first inspection."
- Alan B.
