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 Costa Mesa
Most Costa Mesa additions and ADUs run $45,000-$130,000 depending on scope, and the real variable in this city isn’t labor, it’s what’s hiding inside your 1950s walls. Barrett & Sons Construction has spent 29 years opening up Costa Mesa homes, and we plan around what we know is there: galvanized supply lines, undersized panels, marine-layer rot in crawl spaces. We’re not a franchise dispatching strangers. Tom Barrett and his sons run the crews, every permit gets pulled, and every price change arrives on paper before a single nail moves. For a free estimate on a Home Addition, ADU, garage conversion, or second story in Costa Mesa, call (877) 610-1318.
Why Barrett & Sons Construction Is Costa Mesa’s Preferred Home Additions & ADU Company
Our reputation in Costa Mesa was built one permit inspection at a time. Homeowners on the Eastside know us because we’re the ones who show up when the inspector does, with the panel upgrade already documented and the change order already signed. We answer our own phone, 24 hours a day, and a live person picks up, not a voicemail box or an out-of-state call center. When you call about a room addition off Santa Ana Avenue or a garage conversion near Fairview Park, you’re talking to someone who has actually stood in those houses. That matters when your question is about setback math on a 6,000 sq ft lot, not a scripted sales pitch.
We carry premium brands across every trade: KraftMaid and Cambria in kitchens, GAF and CertainTeen on Costa Mesa Roofing, Andersen and Marvin on windows. And every job ends with the 90-Day Done Right Promise written into the contract: if it’s not done right, Tom comes back and makes it right, one call. That’s the whole deal. No fine print. No arguing about whether a callback counts.
Our Home Additions & ADU Services in Costa Mesa
ADU Design & Permitting
We handle the full Costa Mesa Building Division pipeline: design, structural engineering, plan check, permits, and final sign-off. The Eastside’s signature puzzle is fitting a detached ADU with required setbacks onto a roughly 6,000 sq ft lot that already holds a 1,400 sq ft primary residence. That forces a two-story ADU design or a full garage conversion more often than not, a configuration Costa Mesa’s plan reviewers see so frequently it’s become a local specialty. We’re comfortable in that room. We know the reviewers, and more importantly, we know what they’ll kick back before they kick it back.
A detached ADU in Costa Mesa runs $90,000-$220,000. A garage conversion runs $30,000-$80,000. Those are real numbers for this market, and we’ll put yours in writing before you commit.
Garage Conversion ADU
For many Costa Mesa Eastside and College Park homes, the garage is the ADU. It’s already built, it already has a roof, and converting it avoids the setback problem entirely. What it usually doesn’t have is a foundation deep enough for habitable space, a vapor barrier that isn’t shot, or electrical service that can handle a second dwelling. We open it up, find out what’s actually there, and price the fix before we touch it. No change order without your signature.
New Detached ADU
When the lot allows it, a new detached ADU gives you a clean build: new foundation, new framing, new everything. On Costa Mesa’s 5,000-7,500 sq ft lots, that often means going up instead of out. We regularly design two-story ADUs on Eastside lots where a single-story footprint simply won’t fit the side and rear setback requirements. Costa Mesa’s marine-layer humidity also demands proper ventilation and moisture control from day one, not as an afterthought.
Junior ADU & Interior Conversion
A junior ADU, or a conversion of an existing bedroom, den, or attached garage space, is usually the fastest and least expensive path to a rentable or family unit. It’s also where we see the most unpermitted work from previous owners: bootlegged wiring, missing fire separation, and egress windows that don’t meet California code. We fix it to permit, and we don’t pretend a permit was optional. The inspector works for you, not for us, and that’s why we like him.
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 Costa Mesa
We specify brands you can look up. For hardscape and exterior elements tied to additions and ADUs, that means Belgard and Techo-Bloc pavers, Quikrete for concrete and masonry, and Cambridge Pavers when the job calls for a specific profile. We don’t install mystery brands off a van. If it goes in your Costa Mesa home, it’s a name you can verify, and it’s backed by the manufacturer’s warranty as well as our 90-Day Done Right Promise. Sourcing for Costa Mesa jobs happens locally, which keeps turnaround tight even when plan check drags.
Common Home Additions & ADU Problems We See in Costa Mesa Homes
- Marine-layer rot in unventilated crawl spaces. Costa Mesa sits close enough to the Pacific that morning humidity cycles through from May to August, and unventilated crawl spaces trap that moisture against floor joists and subfloors. A Home Addition that doesn’t address this will see callbacks within two years. We inspect before we build, not after.
- Original galvanized supply lines and undersized electrical panels. The 1950s-1970s wood-frame ranch and bungalow stock east of Newport Boulevard still carries a lot of its original plumbing and electrical. Opening a wall for an addition triggers code upgrades. We write allowances into the scope of work so those discoveries don’t become arguments.
- Setback math that doesn’t fit the lot. An Eastside lot is usually 5,000-7,500 sq ft. A detached ADU needs side and rear setbacks. Many contractors bid a detached ADU that does not fit with the existing 1,400 sq ft home, forcing an unplanned garage conversion or two-story redesign after permits are pulled. We run the setback math before we bid, not after.
- Corroded ferrous fasteners on exterior elements. Coastal air wreaks havoc on steel. Fasteners, brackets, and straps rust faster in Costa Mesa than in inland cities like Anaheim. We spec corrosion-resistant hardware on every exterior connection, including ADU framing and room addition connectors.
Pricing for Home Additions & ADU in Costa Mesa, CA
Here are the line-item ranges for Costa Mesa’s current market. These are what we quote, and they’re what you’ll see on a written estimate from Barrett & Sons.
| Project | Costa Mesa Price Range |
|---|---|
| Home Addition | $45,000-$130,000 |
| ADU Construction | $90,000-$220,000 |
| Garage Conversion | $30,000-$80,000 |
| Kitchen Remodel | $14,000-$45,000 |
| Bathroom Remodel | $9,000-$28,000 |
What moves a Home Addition quote in Costa Mesa isn’t the square footage, it’s what you find when the drywall comes off. A 1950s Eastside home with original galvanized supply lines and an outdated panel will cost more to bring to code than a 1990s build near the 405 corridor. We handle that with allowances written into the contract before we start, and any surprise is priced and approved as a written change order before we proceed. That’s the only way a fixed price stays fixed. Call (877) 610-1318 to schedule a free estimate, and we’ll give you a line-item written quote, not a phone number promise.
We Also Serve Cities Near Costa Mesa
Our Home Additions & ADU crews regularly work the corridor from Costa Mesa out to Newport Beach, Fountain Valley, Huntington Beach, and Santa Ana. Same owner-operated crew, same written change orders, same 90-Day Done Right Promise. If you’re nearby and weighing a Home Addition or an ADU, we’ll bring the same inspection-first process to your door.
Serving Costa Mesa, CA - Our Local Coverage Area
We’re based in the Costa Mesa 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 Costa Mesa
It depends on your lot dimensions, but with a 6,000 sq ft lot and 1,400 sq ft already built, a detached single-story ADU usually will not meet Costa Mesa’s side and rear setback requirements. Most Eastside homeowners end up with either a two-story detached ADU or a full garage conversion. We run the setback math before we bid, so you’ll know what fits before you spend a dollar on plans. Call (877) 610-1318 and we’ll walk your lot with you.
Expect original galvanized supply lines, an undersized electrical panel, and possible marine-layer rot in unventilated crawl spaces. The 1950s-1970s wood-frame ranch stock east of Newport Boulevard carries these conditions routinely. We open the walls and inspect first, write allowances into the contract, and bring any discovery to you as a written change order before work continues. For Bathroom Remodeling - Costa Mesa projects, this same careful process applies.
Costa Mesa’s Building Division processes ADU permits on a predictable pipeline, but plan check comments and inspection scheduling can add weeks, especially on two-story ADUs and garage conversions with structural upgrades. We handle the full pipeline and we’re on a first-name basis with the reviewers. The cost impact is usually limited to plan check fees and any structural requirements the reviewer adds, which we’ll price in writing before you approve.
Undocumented conditions: corroded galvanized supply lines, undersized electrical panels, and rotted framing that wasn’t visible before demo. On a 1950s ranch off Santa Ana Avenue, our crew opened a load-bearing wall and found exactly that. The permit triggered a panel upgrade and full rewire before rough-in could close. Because we had written allowances into the scope, the change order was approved on-site without stalling the draw schedule or the inspection sign-off. That’s how we protect your budget. Call (877) 610-1318 for a free estimate with written allowances.
Yes, we do it regularly on Costa Mesa lots where the garage footprint is the only buildable area left. But an existing garage foundation and framing are rarely engineered for a second story. You’ll typically need a new or reinforced foundation, upgraded shear walls, and possibly a lateral upgrade for the whole structure. We’ll give you the full structural picture in writing before you decide. Call (877) 610-1318 for a free, no-obligation assessment.
Let’s Look at Your Costa Mesa Home Together
Opening up a 1950s Costa Mesa home is the only way to know what you’re really building on, and that’s exactly why we start with an inspection, not a handshake and a verbal number. Tom Barrett, his sons, and the Home Additions & ADU team serve all of Home Additions & ADU in Costa Mesa, including 92626, 92627, and 92628. Every scope, every allowance, every price change arrives as a written change order before any work happens. Permits pulled. Inspections signed. Ninety days, done right, in writing.
Call (877) 610-1318 to schedule your free estimate. Ask for Tom. He’ll answer.
Written by Tom Barrett, Owner at Barrett & Sons Construction, serving Costa Mesa since 1998.