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 Marino
Home additions and ADU construction in San Marino typically run $45,000-$130,000 for a room addition and $90,000-$220,000 for a new detached ADU. Garage conversions land between $30,000-$80,000. Those are the honest market ranges for this city, where a 1920s Spanish Colonial or 1930s Tudor can’t just be copied and pasted with modern materials. We’re Barrett & Sons Construction, and we’ve been building and remodeling in Southern California since 1998. If you’re planning an ADU or second story addition in San Marino, call (877) 610-1318 for a free estimate and a straight answer about what your project will actually take.
San Marino isn’t a generic construction market. The homes are historic, the Design Review Board is exacting, and the lot lines on St. Albans Road or near Lacy Park leave little room for mistakes. Most of the work here is high-end residential renovation of 1920s-1940s estate homes. That changes everything about how a good contractor approaches an ADU. We’ve learned it the hard way: the contractor who shows up with a generic bid loses six months and a deposit. The contractor who knows the difference between Ludowici tile and asphalt shingles gets the job.
Why Barrett & Sons Construction Is San Marino’s Preferred Home Additions & ADU Company
Tom Barrett started this company in 1998, and he still puts his name, his address, and his license number on every contract. That’s rare in this business. Most contractors hide behind a phone number and an LLC. Tom doesn’t. When we work in San Marino, our crews are led by Tom’s sons, and the guy who signed your contract is the guy who will show up if something isn’t right. The Home Additions & ADU team at Barrett & Sons has navigated the San Marino Design Review Board enough times to know what gets approved and what gets rejected.
We’ve earned our reputation in San Marino the slow way: one completed project at a time. Customers here tend to ask a lot of questions, and they should. They’ve usually been burned before by a national company that quoted one price and delivered another. Our 90-Day Done Right Promise is written on every contract, not hidden in a footnote. If something isn’t done right, one call brings Tom back. Every price change comes as a written change order before any work happens. Every eligible job gets permits pulled and inspections signed. That’s not marketing language. That’s how we’ve stayed in business since 1998.
Our Home Additions & ADU Services in San Marino
ADU Design & Permitting
This is where San Marino projects live or die. The city’s Design Review Board requires that any addition or ADU replicate the original home’s roofline pitch, window trim, and exterior finish material. Contractors who show up with generic stucco or asphalt shingles on a 1930s Spanish Colonial lose bids to those who already know Ludowici tile profiles and lime-based stucco formulas. We sit down with you before drawings ever start and map out what the DRB will accept based on your home’s original architecture. That saves you months of rejection letters and thousands in redesign fees. Call (877) 610-1318 to start with a free consultation.
Garage Conversion ADU
Most San Marino garages from the 1920s and 1930s are detached structures with wood siding or stucco that matches the main house. Converting one to an ADU usually costs $30,000-$80,000 in this market, depending on whether you’re doing a clean interior build-out or a full structural upgrade. You’ll also need to account for seismic work. Many of these garages have unreinforced masonry foundations that predate modern codes. We handle the structural engineering, the permits, and the DRB submittal as one package. A live person answers our phone 24 hours a day if you want to talk through what your garage can support.
New Detached ADU
Building a new detached ADU in San Marino runs $90,000-$220,000. The range is wide because the DRB will require the new structure to match the main house’s architectural character, and that drives material costs significantly. A simple 400-square-foot unit with standard finishes sits at the lower end. An 800-square-foot ADU with clay tile roofing, period millwork, and a foundation engineered for the Sierra Madre fault zone sits at the upper end. We don’t give you a number until we’ve walked your lot and reviewed your home’s roofline. What we will give you is a written, line-item scope before any work begins.
Junior ADU & Interior Conversion
A Junior ADU (JADU) is usually the fastest path to adding a rentable or in-law unit in San Marino. These conversions carve space out of an existing home, typically a large bedroom wing or a den with its own entrance. Because the exterior of the home usually isn’t touched, DRB review is often shorter, though an interior conversion that adds a kitchen or plumbing still needs building permits and inspections. We’ve converted former maids’ quarters in 91108 estates into full JADUs with separate entries, kitchenettes, and their own address on the utility panel. The owner’s sons run these crews, and every change stays in writing.
Second Story Additions
Adding a second story in San Marino is one of the hardest jobs in residential construction. The original framing in most of these homes is old-growth Douglas fir, which is strong but was never sized for a second floor load. Before we draw anything, we order a soils report and evaluate the existing foundation and cripple walls. Many San Marino homes sit on the alluvial fan below the San Gabriel Mountains, near the Sierra Madre fault zone. A plan that ignores that isn’t a plan. It’s a demolition order waiting to happen. We engineer for it from day one.
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 Marino
For hardscape and site prep around ADU foundations and patio additions, we carry Techo-Bloc and Cambridge Pavers, plus Quikrete products for concrete and masonry work. These are brands you can look up. When we pour an ADU foundation or build a paver path from the main house to a new detached unit on a San Marino lots, we use materials that can be verified by name. If a contractor hands you a bid that says “concrete” and “pavers” without naming what you’ll get, that’s a red flag. We name everything. Every allowance, every material, every change.
Common Home Additions & ADU Problems We See in San Marino Homes
- DRB plans with the wrong roof profile. A contractor submits a 4:12 asphalt shingle roof on a home where the original 5:12 Ludowici clay tile sets the block’s character. The result is months of rejection letters and a homeowner who’s paid for drawings the city won’t approve.
- Skipping the seismic evaluation. Starting foundation work for a second story addition without a soils report or seismic retrofit of existing cripple walls is a common surprise on San Marino’s alluvial fan near the Sierra Madre fault zone. We don’t skip it, and you shouldn’t let anyone else skip it either.
- Voiding a builder warranty. Touching a shared wall or roof during an ADU addition before the warranty period expires, without written sign-off from the original builder, can void your coverage. We’ve had to walk homeowners back from this more than once. The paperwork is the job.
- Santa Ana wind damage to aging tile roofs. San Marino’s proximity to the mountains channels strong Santa Ana wind events that accelerate deterioration of original clay tile roofs. When we add an ADU that ties into an existing roof line, we always inspect the original tiles and flashing. A new ADU attached to a failing roof is a future insurance claim, not a finished project.
Pricing for Home Additions & ADU in San Marino, CA
Here’s what you’re actually looking at for Home Additions & ADU in San Marino. These are the ranges we’ve quoted and built in this market, not numbers pulled from a national cost calculator.
| Project Type | Typical Range |
|---|---|
| ADU Construction | $90,000-$220,000 |
| Garage Conversion | $30,000-$80,000 |
| Home Addition | $45,000-$130,000 |
What moves the needle is architectural matching. A detached ADU on a 1930s Mediterranean Revival in the 91118 zip will cost more than the same square footage in a city without historic review, because the roof tile, stucco formulation, and window trim have to match the original home. That’s not a premium we invent. It’s what the San Marino Design Review Board requires. We write every scope and every allowance in writing before work starts. No surprises at the end. Call (877) 610-1318 for a free estimate and a real price for your specific lot.
We Also Serve Cities Near San Marino
In addition to San Marino, we build room additions and ADUs in San Gabriel, East San Gabriel, Alhambra, and East Pasadena. The same crew, the same written change orders, the same 90-Day Done Right Promise. If you’re in one of the surrounding cities, call (877) 610-1318 and we’ll schedule a site visit.
Serving San Marino, CA - Our Local Coverage Area
We’re based in the San Marino 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 Marino
Yes. The Design Review Board requires any addition or ADU to replicate the original home’s roofline pitch, window trim, and exterior finish material. If your main house has original or compatible Ludowici-style clay tile in a 5:12 pitch, your ADU needs the same profile or a DRB-approved compatible replacement. We’ve seen plans rejected over a half-degree roofline difference. Call (877) 610-1318 and we’ll tell you exactly what the DRB will expect on your street.
Sometimes, but not always. If the ADU shares a wall or roofline with the main house, you’ll likely need a structural engineer’s assessment. Many San Marino homes have unreinforced masonry and cripple walls that don’t meet current seismic code near the Sierra Madre fault zone. A detached garage conversion is often simpler. We’ll give you a straight answer after a site visit, not before, because every home’s foundation is different. Estimates are free.
It typically adds two to six months, depending on the submittal’s completeness and the DRB’s meeting calendar. The fastest way through it is to submit plans that already match the home’s architectural character: correct roofline, correct window trim, period-appropriate exterior finish. Plans that look generic get tabled, commented, and resubmitted. That’s why our ADU design team includes the DRB review from the first sketch, not as an afterthought. Call (877) 610-1318 to talk through your timeline.
Only if the original builder signs off in writing before you touch any shared wall, roof, or foundation. We handle that paperwork as part of the pre-construction scope. If a contractor starts cutting into your home without that sign-off, you’re gambling with your warranty. We don’t gamble with other people’s coverage. The sign-off goes in writing with your contract, and every change after that comes as a written change order.
Inspect it before you add anything. San Marino’s mountain proximity channels strong Santa Ana wind events that loosen aging clay tiles and accelerate flashing failure. If you’re tying a new ADU roofline into an original tile roof, the old roof needs to be sound first. We include a roof inspection on every ADU and addition that connects to an existing roofline. If repairs are needed, they go into the written scope as a line item, never as a surprise invoice. Call (877) 610-1318 for a free estimate that includes the roof check.
Written by Tom Barrett, Owner at Barrett & Sons Construction, serving San Marino since 1998.