Free written scope consultation - allowances, timeline, and change-order rules on paper before a single trade is scheduled
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AfterFast, Reliable Home Additions & ADU Across San Lorenzo
Home additions and ADU construction in San Lorenzo typically cost $45,000 to $130,000 for a room addition and $90,000 to $220,000 for a detached ADU, with garage conversions landing between $30,000 and $80,000. Those are real numbers for this market, not a national average. If you live in one of the Bohannon tract homes between Hesperian Boulevard and the creek, you’re dealing with a specific set of challenges: 70-year-old framing, expansive clay soil, and a homeowners association that gets a say before the county does.
We’ve built additions and ADUs across San Lorenzo for years, and we know the two-step approval dance here better than most. Call (877) 610-1318 and Tom or one of his sons will walk you through exactly what your project needs, including the SLVHA review most contractors forget to mention until it’s too late.
Why Barrett & Sons Construction Is San Lorenzo’s Preferred Home Additions & ADU Company
We’re the owner-operated alternative to outfits that quote one number on the phone and show up with another. Tom Barrett founded this company in 1998, and his sons run the crews. When we give you a price for a San Lorenzo room addition, it’s in writing. When something changes, you get a written change order before anyone swings a hammer. That’s not a sales pitch. It’s how we’ve kept our name clean for over 29 years.
San Lorenzo is different from Hayward and San Leandro. The San Lorenzo Village Homes Association wraps almost every exterior project in a second layer of review, and we’ve sat through enough Architectural Control Committee meetings to know what gets approved and what gets sent back. We file the SLVHA paperwork alongside the county permits so the calendar doesn’t stretch out while a committee deliberates.
Our customers here don’t want a franchise dispatching strangers. They want a face they’ll see at the hardware store on Hesperian. Our crew members are background-checked, uniformed, and they carry the Barrett & Sons name. A live person answers the phone 24/7. And every job, every addition, every ADU, is backed by our written 90-Day Done Right Promise.
Our Home Additions & ADU Services in San Lorenzo
We offer Home Additions & ADU services that cover the full scope of what San Lorenzo homeowners actually need. That starts with the paperwork and runs through final inspection. Here are the four we handle most in 94580.
ADU Design & Permitting
In San Lorenzo, design and permitting is a two-track process: SLVHA Architectural Control Committee approval first, then Alameda County building permits. We prepare the full submittal packet, including the architectural drawings, materials schedule, and color selections the ACC wants to see. For a typical detached ADU, the SLVHA review adds roughly four to eight weeks to the calendar. We schedule excavation and site prep around that window so you’re not paying for an idle crew.
Garage Conversion ADU
Converting a garage in a 900-to-1,100-square-foot Bohannon tract home is the most cost-effective way to add living space in San Lorenzo. Interior conversions generally don’t trigger the same exterior design review as a detached structure, but the county still wants the full permit set: engineering, electrical, insulation, and egress. Our crews have opened enough of these 1940s garages to know the slab might need leveling and the walls probably need new shear panels.
New Detached ADU
A detached ADU on a San Lorenzo lot means a new foundation, and that foundation is sitting on East Bay expansive clay. We order a soils report before we draw the slab, and if the report calls for piers, we pour piers. Skip that step and you’ll see cracks in the stem wall within two wet-dry seasons. We also handle the site prep and utility rough-ins: water, sewer, gas, and a new electrical subpanel sized for the ADU’s load.
Junior ADU & Interior Conversion
A junior ADU, typically a bedroom with a kitchenette inside the existing footprint, is the fastest path to a rental unit in San Lorenzo. Interior-only JADUs usually skip the SLVHA exterior review entirely, which saves weeks. The county still wants fire separation, a separate entrance, and a permit. We handle the full conversion, including the required smoke and CO detectors and the sound attenuation between the main house and the unit.
What happens when you call
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A real person answersNo phone trees - you reach a local pro.
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You get an upfront price rangeHonest numbers before anyone is dispatched.
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A background-checked tech heads outLicensed & insured, dispatched right away.
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You approve before work beginsNothing starts until you say go.
Trusted Brands We Service in San Lorenzo
We install windows and doors from Simonton, Ply Gem, and PGT on San Lorenzo additions and ADUs. These are brands you can look up, not mystery products off a van. For a second-story addition on a Bohannon tract home, we typically recommend Simonton double-hungs because the proportions match the original 1940s windows the SLVHA’s Architectural Control Committee expects to see in its submittals. For a detached ADU with a modern aesthetic, Ply Gem’s contemporary lines hold up well against the bay winds. We keep local inventory for common sizes so a broken sash or a mis-measured opening doesn’t stall your rough-in.
Common Home Additions & ADU Problems We See in San Lorenzo Homes
- Undersized 1940s framing behind walls. Bohannon cut corners in the immediate postwar rush. We routinely find floor joists at 24-inch spacing instead of 16, and non-standard stud layouts. Any addition that ties into the existing structure needs a structural engineer to look before framing begins.
- Expansive clay soil shifting slabs. The soil under San Lorenzo shrinks in the dry months and swells when the rains come. A new ADU slab poured without a soils report will move. We’ve seen misaligned framing and cracked foundations within two seasons on jobs where a contractor skipped the geotech.
- Cease-and-desist letters from the SLVHA. Starting work without Architectural Control Committee approval gets you a stop order, even if the county already issued the permit. We have watched neighbors rip out fresh stucco because the ACC never signed off on the color.
- Original wiring and plumbing rough-ins at end of life. Many of these homes still have early Romex or the last remnants of knob-and-tube in the attic. When you add a second story, the county requires the existing systems to be brought up to code. That’s a real line item, and we put it in writing before we start.
Pricing for Home Additions & ADU in San Lorenzo, CA
We publish ranges because our customers in San Lorenzo tell us they’re tired of calling three contractors and getting three wildly different numbers, none of them written down. Here’s what we see as real, market-calibrated pricing for this area.
| Project Type | Typical Range in San Lorenzo |
|---|---|
| Home Addition | $45,000 – $130,000 |
| ADU Construction | $90,000 – $220,000 |
| Garage Conversion | $30,000 – $80,000 |
What changes the number: whether your addition ties into the existing structure or stands alone, what the soils report says, what the SLVHA requires for exterior materials, and what condition your 1940s electrical panel is in. We’ll give you a written scope with allowances in writing. If a city inspector or the ACC requires something we didn’t anticipate, you’ll get a change order on paper before that work starts, not a surprise invoice at the end. Call (877) 610-1318 to schedule your free estimate.
We Also Serve Cities Near San Lorenzo
Our crews live and work throughout the East Bay, and we regularly take jobs in Ashland, San Leandro, Cherryland, and Castro Valley. If you’re a few blocks outside the 94580 border, we’ll still show up on time and with the same written-scope guarantee. Call us and ask.
Serving San Lorenzo, CA - Our Local Coverage Area
We’re based in the San Lorenzo 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 - Home Additions & ADU in San Lorenzo
Interior garage conversions that don’t alter the exterior appearance generally do not require San Lorenzo Village Homes Association Architectural Control Committee review, but they still require full Alameda County building permits, including engineering for the new living space and electrical upgrades. Exterior changes like adding windows, moving a door, or changing the roofline do trigger SLVHA review. Call us with your address and we’ll tell you which track your project falls on before you spend a dime.
In our experience, the SLVHA Architectural Control Committee typically adds four to eight weeks to a project, though we have seen faster turnarounds when the submittal packet is complete and the design matches Bohannon-era character. A submittal designed with a flat roof and modern stucco will get sent back, and that redesign costs you weeks. We file the packet correctly the first time and schedule our crews around the approval window rather than letting the job sit half-finished.
The SLVHA’s Architectural Control Committee leans toward low-slope roofs matching the original Bohannon-era profile, wood siding or period-appropriate stucco in approved colors, and window proportions that echo the 1940s originals. Modern flat roofs and oversized modern window walls have been rejected on tract homes in our experience. We prepare submittals using materials like Simonton double-hung windows that align with the committee’s expectations and avoid the redesign loop altogether.
No. Starting site work or foundation work before the SLVHA Architectural Control Committee signs off exposes you to a cease-and-desist order even if Alameda County has already issued a permit, and we have seen this happen off Paseo Grande and along Bockman Road. The two-track approval is sequential: SLVHA first, county second, then we mobilize. We plan the schedule so foundation work starts the week the last signature lands.
Yes, and we won’t pour a slab here without one. The expansive clay beneath San Lorenzo moves enough between the dry summer and the wet winter to crack a slab and misalign framing within two seasons. The county will often require a geotechnical report for a detached ADU anyway, and even when it doesn’t, we order one as a matter of doing the job right. The cost of a soils report is a line item you’ll see in writing on your first scope.
Call (877) 610-1318 to schedule your free estimate. A live person answers, day or night, and we’ll set up a time that works for you. If you have a written estimate from another contractor, bring it in for a free second opinion. We serve the entire 94580 area, and we’ll walk your property with you before we put a single number on paper.
Written by Tom Barrett, Owner at Barrett & Sons Construction, serving San Lorenzo since 1998.