A free site visit, a check of what your lot allows and a written, line-item price.
Get your free estimateFast, Reliable Home Addition Contractor Across Kensington
Adding a room to a home in Kensington isn’t a flat-lot job. The hillsides, the county permitting, the winding streets off Arlington Avenue and Purdue Avenue all shape how the work gets done and what it costs. We’re the Home Addition Contractor team at Lodgewick ADU San Francisco, and we’ve scoped enough additions in this unincorporated community to know that the first conversation should cover county code overlays, access planning, and a written price before anyone moves a shovel. If you’re planning a bathroom addition, bedroom addition, or a larger bump-out on your Kensington hillside, call (707) 760-0502 for a free estimate. We’ll give you a real number, not a vague range that changes later.
Why Lodgewick ADU San Francisco Is Kensington’s Preferred Home Addition Contractor Company
Kensington homeowners have told us the same story more times than we can count: a contractor quoted one price, started the job, then the cost crept up when the hillside or the county requirements got in the way. That’s exactly what our written pricing prevents. Russ Delmar, who leads our ADU Builders - Kensington team, spends his time on Arlington Avenue, Berkeley Park Boulevard, and the streets off Grizzly Peak helping homeowners understand what an addition will actually take.
We work under The Kingswell Guarantee, which means a few specific things. The price is written before work begins. The crew shows up in the window we give you. The job site gets cleaned at the end of every day. And the work carries a 365-Day Done Right Promise: if something’s not right, we come back and make it right. That commitment goes in writing on every Kensington project, including free return visits if the work fails within the guarantee period.
We also answer the phone. A live person picks up 24 hours a day, 7 days a week. No voicemail loops when you have a question about a foundation issue or a county inspection. For Kensington specifically, our scheduling is coordinated from the brand’s San Francisco Bay Area office, with crews dispatched to Kensington homes directly.
Bathroom Addition Contractor in Kensington
A bathroom addition in Kensington typically runs $35,000 to $75,000, with the biggest variables being foundation work on sloped lots, plumbing rough-ins through mid-century framing, and whether the new bathroom ties into existing drain lines or needs its own run to the county sewer line. When a Kensington homeowner needs a second full bath for a growing family or an aging parent moving in, we scope the job against Contra Costa County’s hillside setback rules before quoting. On lots off Purdue Avenue or along Arlington, that often means a stepped footing or pier support rather than a flat-lot slab.
Hillside Plumbing and Drainage
Adding a bathroom on a Kensington hillside means thinking about where the water goes. We’ve installed curtain drains alongside new bathroom additions to keep wet-winter groundwater from pushing against the foundation. The slope itself often dictates drain line placement, and we map that before we quote, so the written price reflects the actual pipe run.
Fire-Hardened Materials
Kensington sits in the East Bay hills’ high wildfire-risk zone, and the county enforces fire-resistant siding and ember-resistant vents on new additions. We include those in the quote up front, because a bathroom added with the wrong materials won’t pass county inspection and will cost more to fix later.
Bedroom Addition Contractor in Kensington
A bedroom addition in Kensington generally runs $50,000 to $110,000, driven mostly by the foundation engineering a hillside lot requires and the cost of bringing the room up to current county energy and fire codes. When homeowners in the Berkeley Park neighborhood or off Grizzly Peak Boulevard need another bedroom for a child or a home office that doubles as a guest room, we start with a soils and slope assessment. A mid-century home on a steep grade almost always needs a stepped or pier foundation, and that’s a bigger cost than the framing itself.
Slope Engineering and Setbacks
Contra Costa County requires a minimum setback from any septic tank and leach field, and on a Kensington lot those systems are often closer to the house than a flat-lot build would allow. We’ve moved leach field lines and built on piers to bridge the extra distance without destabilizing the slope. That’s a local reality, and it’s in our written price, not a surprise later.
Light, Air, and Vegetation
Many Kensington bedroom additions sit under mature oaks and eucalyptus. We plan window placement and ventilation for fire safety and natural light, trimming defensible space per county requirements. A bedroom that feels open and bright here also has to be built to survive a dry, hot fire season, and we handle both.
Trusted Brands We Service in Kensington
When we build additions in Kensington, we work with materials and systems we’d put on our own homes. We’ve framed with James Hardie siding for its fire resistance and durability on exposed hillside walls. For detached additions and studio-style bump-outs, we’ve specified Plant Prefab and Tuff Shed structures where the lot and county rules allow. The point isn’t the brand name. It’s matching the right material to a steep, fire-prone lot and getting it delivered up a road a concrete truck can’t always manage. We stock commonly needed components for Kensington projects and can pivot quickly when a county inspector flags a material spec.
Common Home Addition Contractor Problems We See in Kensington Homes
- Hillside erosion-control violations. A contractor pulls a flat-lot permit and ignores Kensington’s erosion-control ordinance. The first winter rain washes silt onto Colusa Avenue, and the county slaps a stop-work order on the job. We include erosion control in every hillside scope from day one.
- Septic and well setback misses. An addition gets framed over an old septic tank without checking Contra Costa County’s required 10-foot setback from the new foundation. The homeowner faces a costly tank relocation mid-project. We map septic and well locations before any framing plan is drawn.
- Access and traffic-control fines. A concrete truck blocks a blind curve on Grizzly Peak Boulevard while pouring an addition slab, and the county road crew fines the contractor for an unpermitted traffic-control plan. Every Kensington project starts with a traffic and logistics plan for narrow roads and steep driveways.
- Foundation surprises on mid-century builds. Kensington’s stock is mostly 1950s-era homes on original foundations. Opening a wall often reveals settling, cracked footings, or undersized piers that a flat-lot contractor wouldn’t expect. We scope for that reality and price accordingly.
Pricing for Home Addition Contractor in Kensington, CA
Here are honest numbers for this market. A bathroom addition in Kensington runs $35,000 to $75,000. A bedroom addition runs $50,000 to $110,000. A detached home addition on the flatter part of a lot, if septic and well setbacks allow, can start around $65,000 and climb depending on foundation type and fire-code materials. Hillside work adds cost: stepped footings, pier foundations, drainage, and erosion control can add $12,000 to $30,000 over a flat-lot build. Access matters too. If your Kensington street can’t fit a concrete pump or mixer, we may need a transfer truck or pump line, and that’s in the written quote before work starts. Call (707) 760-0502 for a free, itemized estimate. No change orders at the door.
We Also Serve Cities Near Kensington
Our crews regularly work in Kensington and the surrounding East Bay. We also serve homeowners in San Francisco, Oakland, Hayward, and Richmond, applying the same written pricing and county-specific scoping to every hillside addition. No two lots are the same, and we don’t price them that way.
Serving Kensington, CA - Our Local Coverage Area
We’re based in the Kensington 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 About Home Addition Contractor in Kensington
Yes, in most cases. Contra Costa County reviews all hillside development for grading and erosion control, even when the addition sits on a stepped footing rather than a cut slope. The county’s hillside ordinance applies to any soil disturbance beyond a minimal threshold, and additions on Kensington’s sloped lots almost always trigger a grading review. Call (707) 760-0502 and we’ll tell you what your specific lot will need before you apply for anything.
Contra Costa County requires a minimum 10-foot setback from any new foundation to a septic tank, and the leach field has its own larger setback depending on soil type and slope. Kensington’s older homes often have septic systems closer to the house than modern code allows, which means an addition may require relocating the tank or shifting the leach field. We scope this before quoting so the price reflects reality, not a surprise. Call (707) 760-0502 for an exact assessment on your lot.
Yes. Kensington sits in the East Bay hills’ high wildfire-risk zone, and Contra Costa County enforces fire-resistant siding and ember-resistant vents on new additions. Materials like James Hardie fiber cement meet the county’s requirements, and we include them in the written quote. Skipping them means failing inspection and paying to replace non-compliant work. Call (707) 760-0502 to get a quote that includes fire-hardened materials from the start.
It depends on your septic system’s capacity and the county’s setback rules for the leach field. A detached addition on the flatter part of a Kensington lot still needs to connect to the existing septic or tie into the county sewer line if one runs near the property. The county won’t approve a habitable addition on an undersized septic system. We map septic and sewer lines early and price the connection work in the written estimate. Call (707) 760-0502 to schedule a free site review.
The biggest issue is getting a concrete mixer or crane to the site. Kensington’s winding, narrow roads with tight turnarounds and steep driveways mean a standard mixer often can’t reach the pour. We plan for concrete pumps, transfer trucks, or staged deliveries, and that access plan is priced in the written quote before work starts. Call (707) 760-0502 and we’ll walk your street and lot to see what’s needed.
Building on Kensington’s Hillsides: County Rules Most Contractors Miss
Kensington is unincorporated. That’s not a technicality; it changes everything about how a home addition gets permitted and built. Contra Costa County, not a city, reviews every addition for hillside development, septic and well setbacks, and wildfire defensible space. A homeowner in Berkeley or El Cerrito might deal with a city planning desk and flat-lot rules. In Kensington, the county’s hillside ordinance applies to almost every addition, even when you’re not cutting into the slope. Our Home Addition Contractor team scopes to county code overlays from the first site visit onward. That means checking septic tank setbacks before drawing foundation lines, planning erosion control before the winter rains hit, and specifying fire-hardened siding and vents from the beginning. A contractor who pulls a flat-lot permit and ignores these overlays gets a stop-work order, and the homeowner pays for it. We don’t let that happen.
One job off Purdue Avenue shows what this actually looks like. We added a primary suite above a 1950s foundation on a mid-century sloped lot. Our crew re-graded the hillside for a stepped footing, installed a curtain drain to keep wet-winter groundwater off the new slab, and routed a concrete pump through a neighbor’s easement because the street’s 18-foot turnaround couldn’t fit a mixer. The county’s well setback moved the septic leach field, so we built the addition on piers to bridge the extra 14 feet without touching the slope. That’s Kensington work. None of it appears on a generic home addition estimate, and all of it was in the written price before we started.
The steep-hillside, unincorporated reality shapes every decision. Wet winters mean erosion control isn’t optional. Dry summers mean fire-hardened materials aren’t an upgrade; they’re code. And the winding, narrow roads off Arlington and Grizzly Peak mean every project starts with a traffic and logistics plan. Getting a concrete truck or crane to the site can be the hardest part of the job. We plan for that before quoting, which is why our written price reflects the actual cost of building on this terrain. If you’re adding to a Kensington home, call (707) 760-0502 for a free estimate, and we’ll explain what your lot needs before you commit to anything.
Contact Lodgewick ADU San Francisco for Your Kensington Home Addition
If you’re planning a bathroom addition, bedroom addition, or a larger bump-out on your Kensington hillside lot, call (707) 760-0502 to schedule a free estimate. Russ Delmar and the Lodgewick crew will walk your property, review county requirements, and hand you a written price before any work starts. The 365-Day Done Right Promise covers every Kensington addition in writing, and a live person answers the phone 24/7 when you have questions. Call today for your free estimate.
Written by Russ Delmar, Owner at Lodgewick ADU San Francisco, serving Kensington and the surrounding East Bay.
