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Our home base — 1950s tract homes on hillside and valley lots. We know this housing stock inside and out.
CSLB #1041528 — Family-Owned, Pacifica-Based
Room Additions & Second-Story Additions on the Peninsula.
Starting with the structure, not the floor plan. We assess your foundation and framing before designing — because the bones of your 1950s home determine what’s possible.
Why Structure Comes First
The Structural Conversation — Why It Comes First on the Peninsula
Most Peninsula homes were built between the late 1940s and early 1970s with foundations and framing sized for a single-story ranch house.
Foundation Capacity
Can existing footings support additional load? Slab foundations often need reinforcement before any new weight is added above.
Wall Framing
Are load-bearing walls adequate for additional weight? 1950s headers and shear walls may not meet current code requirements.
Soil Conditions
What is the bearing capacity? Hillside lots may have fill, colluvium, or expansive clay that limits what the ground can support.
Connection Hardware
Pre-1970s homes lack anchor bolts, hold-downs, and metal connectors that current code requires for structural integrity.
Ground-Level Additions
Expanding Outward — Ground-Level Room Additions
Ground-Level Addition
A ground-level room addition expands your home's footprint outward — adding square footage at the same level as your existing first floor.
You have lot space and setbacks allow expansion
Zoning limits will determine how far from property lines you can build.
You want to stay on one floor
Aging-in-place, accessibility, or avoiding the complexity of second-story construction.
Your foundation can't support a second story
Without major reinforcement, a new floor may not be structurally feasible.
Typical cost: $300–$600 per square foot. A 400-sq-ft primary suite with bath: $160,000–$280,000.
Second-Story Additions
Building Upward — Second-Story Additions
Second-Story Addition
The most structurally demanding residential project. Every pound of the new second floor transfers through existing walls into a foundation that wasn't designed to carry it.
Seismic requirements near the San Andreas Fault
Peninsula construction must meet California's strictest seismic codes.
Height limits (28–35 feet in most residential zones)
New construction cannot exceed local zoning height restrictions.
Hillside lots increase complexity
Retaining walls, erosion control, and foundation requirements escalate costs.
Neighbor relations — views and streetscape impact
Discretionary review may apply; community input can delay approval.
Typical cost: $400–$700 per square foot. An 800–1,000 sq ft full second story: $320,000–$700,000.
Making the Right Choice
Room Addition vs. ADU — Which Do You Need?
Room Addition
Expands your existing home. Connected by doorway. Shares your address, HVAC system, and kitchen. Built for your household — a bigger primary suite, family room, or extra bedroom.
ADU
Independent dwelling unit. Own entrance, kitchen, bathroom, and utilities. Can be rented to tenants or sold separately under AB 1033. A completely self-contained living space.
If you need a bigger primary suite, family room, or extra bedroom — that’s a room addition. If you need a separate living space — that’s an ADU. We build both.
Our Process
How We Build Additions
One team handles everything — design, permitting, and construction under one contract.
Structural Assessment
Evaluate foundation, framing, and soil. For second-story additions, a structural engineer comes before design begins.
Design
Architect and builder on the same team. Structural findings feed directly into the design so plans reflect reality from day one.
Permitting
Complete permit application prepared and submitted. We manage discretionary review for second-story additions.
Construction
Foundation reinforcement, framing, and retaining walls done by our crew. 3–6 months for ground level, 6–12 months for second story.
Finish & Close-Out
Interior finishes, exterior integration, landscaping, final inspections, and certificate of occupancy.
Ground-level additions: 3–6 months. Second-story additions: 6–12 months from permit approval.
Why ACI
Why Peninsula Homeowners Choose ACI
We do structural work ourselves
Foundation reinforcement, framing, retaining walls, and seismic retrofitting — done by our crew, not subbed out.
We start with the engineering, not the fantasy
We bring the structural engineer in first. The design reflects reality from the start — no surprises during construction.
We know these houses
We've opened walls of 1950s tract homes across the Peninsula. Fewer surprises, more accurate estimates.
One team, one contract
Design-build means no finger-pointing between architect, engineer, and builder. One company, one point of accountability.
Family-owned, Pacifica-based
Licensed and insured. CSLB #1041528 (General Building, Class B). We live and work on the Peninsula.
Licensed & Insured
CSLB License #1041528 (General Building, Class B)
Service Area
Serving the San Francisco Peninsula
Based in Pacifica, we build room additions and second-story additions throughout the Peninsula.
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Victorian, Edwardian, and post-war homes with complex permitting.
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Dense Westlake tract housing where going up is often the only option.
- 📍South San Francisco
Mixed-era neighborhoods, some on significant hillside terrain.
- 📍San Bruno
Suburban lots with room for ground-level additions, particularly in Crestmoor.
- 📍Millbrae, Burlingame & San Mateo
Higher-value homes where additions are substantial investment projects.