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 Woodside
Building an addition or ADU on a Woodside parcel is not like building one in Menlo Park or Redwood City. You are dealing with county building services, discretionary design review, fire-hardening mandates, heritage oak setbacks, and hillside clay that moves with every winter storm. A home addition in Woodside typically runs $45,000 to $130,000, and a new detached ADU lands between $90,000 and $220,000 depending on slope, septic, and fire-resistance requirements. If you are planning a room addition, second story, garage conversion, or a standalone ADU on your property, call Barrett & Sons Construction at (877) 610-1318. Tom Barrett and his sons run the crews themselves, and every project starts with a site walk before a single number goes on paper.
The article you are reading was written for Woodside homeowners specifically. Not a template. The fire hazard zone changes what materials we can install. The oak protection rules change where the building can sit. The rain and clay change what goes under the slab. We will walk you through the exact sequence Woodside requires so you do not spend money on plans the Town will not accept.
Why Barrett & Sons Construction Is Woodside’s Preferred Home Additions & ADU Company
We have been owner-operated since 1998, which means the name on the truck is the name on the contract. Tom Barrett answers his own phone, publishes his address and license number, and his sons are on-site most days. When a Woodside homeowner calls about an ADU, they are not getting a call center in another state. They are getting someone who has pulled permits through the Town of Woodside and San Mateo County, sat through Architectural and Site Review Board meetings, and surveyed heritage oaks before drawing a single foundation line.
Our reputation in Woodside is built on written change orders and permits pulled on every eligible job. Most of our Woodside clients come to us because a previous contractor told them permits were optional or quoted a price that grew by forty percent at the door. That is not how we work. The scope goes in writing. The allowances go in writing. Any change order goes in writing before work proceeds. And at the end, the 90-Day Done Right Promise stands behind the whole thing. If it is not done right, Tom comes back and makes it right. One call.
Our Home Additions & ADU Services in Woodside
New Detached ADU
A new detached ADU in Woodside runs $90,000 to $220,000, and the two biggest cost drivers are site prep and fire-hardening materials. Given that nearly all of Woodside sits in a CAL FIRE Very High Fire Hazard Severity Zone, new detached structures must meet Chapter 7A standards: ember-resistant vents, ignition-resistant siding, and multi-pane glazing. Add a steep driveway, a heritage oak on the parcel, and clay soil that expands in wet winters, and the foundation spec changes quickly. We scope pier-and-grade-beam foundations and drainage before quoting, because on a Woodside hillside, the soils report and the oak survey come before anything else.
ADU Site Prep & Utilities
This is the work nobody sees but everything depends on. On most Woodside parcels, site prep means grading, slope stabilization, retaining walls, and utility trenching across terrain that does not cooperate. Septic and well setbacks apply to an ADU the same way they apply to a primary residence. If your existing leach field is close to the proposed footprint, the addition may need a new field or an upgraded tank before the county will sign off. We price this work line by line, in writing, before the project starts. No allowance that is not on paper.
Room Addition
Most Woodside homes were built between the 1950s and 1980s as single-story ranches or split-levels on large lots. Room additions onto these structures typically run $45,000 to $130,000, and the condition of the existing foundation dictates a lot of that number. Many of these older homes sit on perimeter foundations that were never engineered for an addition, so the footings need to be evaluated and often retrofit. If the room addition sits on a sloped portion of the lot, drainage engineering becomes part of the permit package. We do not guess at this. We walk the site, run the numbers, and put the scope in writing.
Junior ADU & Interior Conversion
A junior ADU inside an existing home is often the most direct path to extra living space in Woodside, and it generally costs less than a detached build. You are working within the existing envelope, but you still need a separate entrance, a small kitchen, and fire separation from the main house. In Woodside homes with older wiring and panel capacity, the electrical upgrade often drives the price. We will tell you exactly what needs upgrading before the first day of framing. If the footprint does not move, you skip the oak survey and much of the grading work, which shortens the timeline and the cost.
Garage Conversion ADU
Converting an attached or detached garage into an ADU in Woodside typically runs $30,000 to $80,000. The existing structure gives you a head start, but the work almost always includes new insulation, new foundation tie-downs for seismic code, and fire-hardening the exterior if the structure falls under the Chapter 7A requirements. If the slab is cracked or uninsulated, we address that before framing interior walls. On equestrian properties, we also see detached garages that were never permitted initially; in those cases, the conversion may require bringing the original structure up to code before the ADU permit can be issued.
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 Woodside
When we install windows and doors on a Woodside addition, we use brands you can look up: Simonton, Ply Gem, and PGT. These are not mystery units off a van. Simonton and Ply Gem offer strong thermal ratings for the cool, damp winters up in the hills, and PGT impact-rated units matter more in a fire zone than most homeowners realize, since multi-pane glazing is part of the Chapter 7A hardening requirement for new construction. We will show you the spec sheets and the pricing before anything gets ordered. Every material line goes in the written scope.
Common Home Additions & ADU Problems We See in Woodside Homes
- Heritage oak drip-line surprises. The Town of Woodside protects oak trees with drip-line exclusion zones that can shrink a legal building envelope mid-design. A foundation corner that looked fine on paper may sit inside a protected zone, forcing a full site-plan revision before the Building Department will accept the application. We survey the trees before we draw the plan.
- Fire-hardening scope creep. Most homeowners do not realize that nearly all of Woodside is a Very High Fire Hazard Severity Zone. Chapter 7A compliance means ember-resistant vents, ignition-resistant cladding, and specific glazing, all of which add real cost to an ADU or addition. We quote it up front, not as a surprise line item later.
- Clay soil and seasonal movement. Woodside gets 25 to 30 inches of rain a year, far more than flatland Peninsula neighbors. The clay-heavy hillside soils expand and shift seasonally. Without a slope stabilization plan and proper drainage, a new addition’s subfloor will crack, and the punch list never closes. We engineer drainage into the original scope.
- Septic and well setbacks blocking the build. An ADU is subject to the same septic and well setback rules as a primary residence. On some Woodside parcels, the existing leach field sits too close to the only logical ADU footprint. That means a new field or an upgraded tank, and that changes the number on the estimate. We check this before we quote.
Pricing for Home Additions & ADU in Woodside, CA
| Project Type | Typical Woodside Range |
|---|---|
| Room Addition | $45,000 - $130,000 |
| New Detached ADU | $90,000 - $220,000 |
| Garage Conversion ADU | $30,000 - $80,000 |
What moves the number? Slope and grading. The cost difference between a flat buildable pad and a hillside lot that needs retaining walls can be $20,000 or more before framing starts. Fire-hardening materials add to the cladding, vent, and glazing budget. Septic replacement on an older Woodside property can add $15,000 to $40,000 depending on the system. And if a heritage oak survey forces the footprint to move, the design and engineering fees shift with it. We show you every one of these numbers in writing before work begins. Call (877) 610-1318 for a free estimate. We will walk the property with you.
We Also Serve Cities Near Woodside
While this page is written for Woodside homeowners, Barrett & Sons Construction also handles Home Additions & ADU work in Menlo Park, Redwood City, San Carlos, and Atherton. The permitting and site conditions change from one town to the next, and we will tell you honestly which one applies to your property.
Serving Woodside, CA - Our Local Coverage Area
We’re based in the Woodside 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 Woodside
Does Woodside require a separate fire-hardening review for an ADU addition, or is it bundled with the building permit?
The fire-hardening review is bundled with the building permit process, but it is not a rubber stamp. Because nearly all of Woodside sits in a CAL FIRE Very High Fire Hazard Severity Zone, the plans must meet Chapter 7A standards at submission. That means ember-resistant vents, ignition-resistant cladding, and multi-pane glazing are specified in the plan set before the permit is issued. We prepare these details up front so the review does not stall. Call (877) 610-1318 if you want us to walk your parcel and explain exactly what applies.
Can I dig a foundation near a heritage oak on my Woodside lot if the tree stays healthy?
No, not if the excavation falls within the oak’s drip-line exclusion zone. The Town of Woodside’s heritage oak ordinance legally constrains where foundations and grading can go, and the drip line is measured from the outermost edge of the canopy. The tree’s health does not exempt the setback. On many parcels, this means the ADU footprint must shift, sometimes significantly. We survey the trees before finalizing the site plan so you do not pay for a redesign. For an exact read on your lot, call us for a free estimate.
How do Woodside’s 25-30 inches of annual rain change the cost of a room addition compared to a dry Peninsula city?
It changes the drainage and foundation budget. Woodside gets far more rain than flatland Peninsula neighbors, and the clay-heavy hillside soils expand and shift seasonally. That means a room addition on sloped ground needs engineered drainage, likely retaining walls, and often a pier-and-grade-beam foundation rather than a simple slab. These are not upgrades; they are prerequisites for a permit. Expect the added engineering and site work to account for a meaningful portion of the $45,000 to $130,000 range. We quote it in writing before work starts.
Is an ADU in Woodside subject to the same septic and well setback rules as a primary residence?
Yes. An ADU must meet the same San Mateo County septic and well setback requirements as a primary residence. If your existing leach field sits too close to the proposed ADU, you may need a new field or an upgraded tank before the county approves the building permit. This is one of the first things we check on site, because it can change the project cost by tens of thousands. Call (877) 610-1318 if you want us to review your parcel.
How does Woodside’s Architectural and Site Review Board affect the timeline for a home addition?
The Architectural and Site Review Board must approve the design before the building permit is issued, and this adds weeks to the front end of any Woodside project. It is discretionary review, meaning the board can request changes to massing, materials, or site placement. We prepare submittals that anticipate the board’s concerns, and we file early, because the review calendar does not move for anyone. If you are planning an addition in 94062, start the design conversation sooner than you think you need to. Call us for a free estimate.
Written by Tom Barrett, Owner at Barrett & Sons Construction, serving Woodside and the surrounding Peninsula communities since 1998.