Free written scope consultation - allowances, timeline, and change-order rules on paper before a single trade is scheduled
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Before
AfterFast, Reliable Home Additions & ADU Across San Jose
A typical detached ADU in San Jose runs $106,200-$259,600, while garage conversion ADUs fall between $35,400-$94,400. Most projects we quote in San Jose are scoped, permitted, and underway within 6-10 weeks for city-track ministerial approvals, though county parcels with well and septic review add 4-8 weeks. Call (650) 405-3637 to schedule your free estimate.
We’ve been building in the San Jose market since Rowan Construction Group was founded in 2007, and our Home Additions & ADU team knows the difference between a Trinidad Lane lot in Berryessa with a 10-foot side gate and a Cambrian acreage with county septic setbacks. Our crews are dispatched from our San Francisco Bay Area office, and we serve San Jose properties from Alviso to Alum Rock. The permitting path, the access constraints, and the seismic requirements change by neighborhood here. We scope them all before we write a price.
Why Rowan Construction Group Sunnyvale Is San Jose’s Preferred Home Additions & ADU Company
San Jose homeowners have left us reviews that mention the same thing: the number on the page matched the number on the contract. We’ve delivered 4,000-plus projects across seven trades, every one with a written price before work started and a documented photo record when the crew left. That discipline matters especially in San Jose, where Silicon Valley land values - median home prices consistently above $1.2M - have made ADU construction and garage conversions the single most active residential construction category in the city. A project type that is marginal in most U.S. markets here routinely adds $300K-$500K in appraised value.
Our crews are background-checked and uniformed. A live person answers calls 24/7. And every visit includes a documented photo record as standard, not an upgrade. Under Clause 1 of The Haven Standard, the price is written before anyone starts. Under Clause 3, no change proceeds without a signed change order. That is why our projects finish within a few percent of the agreed cost.
We also offer a Free Second Opinion on any written estimate. Bring a competitor’s quote and we will review it line by line at no charge.
Our Home Additions & ADU Services in San Jose
ADU Design & Permitting
San Jose’s Planning Department processes among the highest ADU permit volumes of any city in California. Contractors who master the city’s specific “ministerial approval” checklist - including the ADU Pre-Approved Plans program - can pull permits in days rather than months. We have. Last spring our crew framed a 24×30 detached ADU off Trinidad Lane in Berryessa where the rear yard sits 140 feet from the street and the only access was a 10-foot side gate. We pulled the ministerial ADU permit in nine days through the city’s pre-approved plan track, then stockpiled framing and set a telehandler to avoid blocking the neighbor’s driveway during the pour. Final inspection sign-off cleared before the drywall hang, and the owner was renting it by late summer.
Garage Conversion ADU
The dominant housing stock in Berryessa, Cambrian, Alum Rock, and East San Jose is 1950s-1970s post-WWII ranch-style tract homes, many with detached garages ripe for conversion. A garage conversion ADU in San Jose typically runs $35,400-$94,400. The price shifts based on whether the existing slab needs seismic retrofit, whether the roof structure can carry insulation and mechanical, and whether the garage sits on a property line that triggers San Jose’s side-yard setback rules. We verify all three before we quote.
New Detached ADU
A new detached ADU in San Jose runs $106,200-$259,600. This is our most-quoted project type in the city, and for good reason. Silicon Valley’s extreme land values make a 400-800 square foot rental unit one of the highest-ROI construction investments available to San Jose homeowners. But the Calaveras Fault runs directly through Alum Rock and the east foothills, placing much of the city in Seismic Design Category D. That means hold-downs, shear wall calculations, and special inspections on virtually every permitted addition. We engineer for this in the initial scope, not as a change order after the first plan check.
Junior ADU & Interior Conversion
Willow Glen’s pocket of 1920s-1940s Craftsman bungalows and cottages presents a different challenge. Remodels there routinely encounter original balloon framing, knob-and-tube wiring rough-ins, and pier-and-beam foundations needing seismic bolting. A Junior ADU - a conversion within the existing footprint, with a separate entrance - can make sense where lot coverage is already maxed. We assess the existing structure before we quote, because a Junior ADU that triggers full seismic retrofit is a different project than one that does not.
ADU Site Prep & Utilities
On acreage around San Jose, county rather than city permitting applies. Septic and well setbacks, long material hauls, and equipment access change the scope before we quote. A 60-foot conveyor, 4-6 crew, and hotshot cars make wide-lot access in Cambrian or Berryessa a scheduling decision before it’s a pricing decision. Bay-fringe areas in North San Jose and Alviso sit on liquefiable bay mud; any new construction or deep foundation work there must address liquefaction risk with geotechnical reports. We flag these requirements in the written scope, not after the permit is pulled.
Room Additions & Second Story Additions
A room addition or second story in San Jose runs $53,100-$153,400. Second stories trigger stricter seismic requirements in SDC D zones and often require foundation reinforcement that a single-story addition does not. We bring a structural engineer to the first site visit when a second story is in discussion. The photo record from that visit goes to the customer within 24 hours, with a written assessment of foundation capacity, shear wall placement, and the likely permitting path.
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.
Before
AfterTrusted Brands We Service in San Jose
We source across fifty-plus vetted brands so the written quote carries honest numbers rather than inflated ones. For window packages in ADU and addition projects, we regularly specify PGT and CGI impact-rated units where the seismic and wind-load calculations warrant them, and CWS and ES Windows for standard efficiency packages that meet California Title 24 requirements. Our San Jose customers see these brands named in the written quote, with model numbers and spec sheets attached. No “or equivalent” language. What we quote is what gets installed.
Common Home Additions & ADU Problems We See in San Jose Homes
- County vs. city permitting confusion. A property on the San Jose fringe may sit in unincorporated Santa Clara County, where well, septic, and outbuilding review adds 4-8 weeks to the calendar. We verify jurisdiction before we quote, because assuming an urban permitting calendar for acreage work throws off the draw schedule and labor sequencing.
- Grading and drainage treated as afterthoughts. San Jose’s expansive clay and foothill slopes mean a failed slope or ponding pad stalls the inspection sign-off. We scope drainage, swales, and cut-fill balance in the initial quote, not as a change order after the first rain.
- Septic and well setbacks unverified. Quoting a fixed price before verifying leach field setbacks or wellhead protection zones means the project gets re-scoped after the first plan check. The change order surprises the owner. We pull county records before we write the price.
- Seismic details missed on older stock. Willow Glen’s Craftsman bungalows and the 1950s-1970s ranch tracts alike may need hold-downs and shear panels that a standard plan set does not address. We send a structural assessor on the first visit when the build date precedes modern seismic codes.
Pricing for Home Additions & ADU in San Jose, CA
Here is what we are quoting in San Jose right now. These are flat-price ranges with written scope and written warranty; “call for a quote” is not an option here.
| Project Type | Price Range in San Jose |
|---|---|
| ADU Construction (detached, site-built) | $106,200-$259,600 |
| Garage Conversion ADU | $35,400-$94,400 |
| Home Addition (room or second story) | $53,100-$153,400 |
| ADU Site Prep & Utilities (grading, drainage, septic/well tie-in) | Quoted per site after assessment |
What moves a project within these ranges: foundation type (slab-on-grade vs. pier-and-beam vs. engineered fill), seismic design category and special inspection requirements, utility tie-in distance and whether the main panel needs upgrade, finish level (builder-grade vs. mid-spec vs. premium), and access constraints that affect equipment and material handling. We assess all six factors on site before we write the price. The quote you receive is the price you pay, plus only signed change orders. Under The Haven Standard, Clause 1 is the whole point.
Call (650) 405-3637 for an exact quote on your project. Estimates are free.
We Also Serve Cities Near San Jose
Our crews build across the South Bay, including Home Additions & ADU in San Jose and neighboring Milpitas, Santa Clara, Sunnyvale, and Campbell. Permitting paths differ by jurisdiction - Santa Clara’s ADU ordinance, Milpitas’s hillside development standards, Campbell’s lot coverage calculations - and we maintain current checklists for each. The written price reflects the actual requirements where you build.
Serving San Jose, CA - Our Local Coverage Area
We’re based in the San Jose 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 Jose
It depends on whether your parcel sits within San Jose city limits or unincorporated Santa Clara County. County parcels require well and septic review through the County Environmental Health Department, plus building permit through the County Planning and Development Department, adding 4-8 weeks to the typical city ministerial timeline. We verify jurisdiction and pull the appropriate records before we quote. Call (650) 405-3637 and we will check your parcel on the first call.
A detached ADU in San Jose runs $106,200-$259,600 for a site-built structure. The range reflects foundation type, seismic requirements for your specific zone, utility run distance, and finish specifications. We deliver a written line-item quote after a site assessment, not a ballpark over the phone. Call (650) 405-3637 to schedule your free estimate.
Construction costs $35,400-$259,600 depending on type, and in San Jose’s market a permitted ADU typically adds $300,000-$500,000 in appraised value. That is not a projection; it is what we see on appraisals for completed projects in Berryessa, Cambrian, and Willow Glen. The value add is specific to Silicon Valley’s extreme land values and rental demand. Call (650) 405-3637 and we will review comparable sales in your neighborhood.
Yes, if the addition respects leach field setbacks, wellhead protection zones, and any recorded easements. County parcels near San Jose often have 50-100 foot setbacks from septic components and 100-foot wellhead buffers. We pull the as-built septic diagram and well location from County Environmental Health before we design, so the footprint we quote is the footprint that gets approved. Call (650) 405-3637 to start the records check.
We engineer cut-fill balance, drainage swales, and compaction specifications into the initial scope, with geotechnical review where San Jose’s expansive clay or liquefiable bay mud requires it. A failed slope or ponding pad stalls inspection sign-off, so we do not treat this as an afterthought. The grading plan, drainage calculations, and any required geotechnical report are line items in the written quote. Call (650) 405-3637 for a site-specific assessment.
Written by Grant Rowan, Owner at Rowan Construction Group, serving San Jose since 2007.