Home Addition in Portola Valley
Custom Home builds home additions in Portola Valley at $600-$850 per square foot. Whether you need a second-story addition, ground-floor expansion, or bump-out, our two-phase process delivers 3D visualization and itemized pricing before construction starts.
How much does a home addition cost in Portola Valley?
Home additions in Portola Valley typically cost $600-$850 per square foot in 2026. A 500 sqft addition ranges from $300K to $425K. Custom Home's design phase locks in your exact cost before construction starts.
Why Expand Your Portola Valley Home
Portola Valley’s strong property values and desirable neighborhoods make home additions a smart investment. Rather than uprooting to a new location, expanding your current home lets you stay in the community you chose. Established neighborhoods and proximity to both San Francisco and Silicon Valley create consistent demand for quality construction.
Custom Home designs and builds home additions with the same two-phase process we use for custom homes. You see the complete addition in 3D and receive itemized pricing before construction begins.
Types of Home Additions We Build in Portola Valley
Second-Story Additions
Double your living space by building up rather than out. Second-story additions are ideal when lot coverage limits prevent ground-floor expansion. Custom Home’s structural engineers evaluate your existing foundation to determine what reinforcement is needed.
Ground-Floor Room Additions
Extend your home’s footprint with new bedrooms, family rooms, or home offices. Ground-floor additions integrate with your existing layout and maintain single-level living, which is especially valued in Portola Valley.
Bump-Outs and Extensions
Smaller expansions that make a significant difference: extend your kitchen, enlarge a bathroom, or add a sunroom. Bump-outs typically cost less and complete faster than full additions.
Structural Considerations for Portola Valley Additions
Portola Valley is located in a seismically active region, which means home additions must meet current seismic codes. Custom Home’s engineering team handles foundation evaluation, load-bearing wall modifications, and seismic design for every addition project. Whether you are adding a second story or expanding your footprint, structural engineering is included in our Phase 1 design process.
Portola Valley Home Addition Permits
Portola Valley’s Building Department processes residential permits with oversight from the Architectural and Site Control Commission (ASCC). All new homes and significant additions require ASCC approval, which evaluates site integration, visual impact, and environmental sensitivity. Plan review typically takes 6-10 weeks. Geotechnical and seismic reports are required for most hillside properties.
Custom Home manages the entire addition permitting process, including structural engineering review, plan submittal, and inspection coordination. Our experience with Portola Valley’s requirements helps keep the approval process on track.
Pricing for Home Addition in Portola Valley
Home addition costs in Portola Valley typically run $600-$850 per square foot, similar to new construction pricing. Second-story additions may cost more due to structural reinforcement requirements. Custom Home's Phase 1 design process locks in exact pricing before construction begins.
Portola Valley Permit Requirements
Portola Valley's Building Department processes residential permits with oversight from the Architectural and Site Control Commission (ASCC). All new homes and significant additions require ASCC approval, which evaluates site integration, visual impact, and environmental sensitivity. Plan review typically takes 6-10 weeks. Geotechnical and seismic reports are required for most hillside properties. Custom Home manages the full addition permitting process, including structural engineering review.
Home Addition in Portola Valley: FAQ
How much does a home addition cost in Portola Valley?
Home additions in Portola Valley typically cost $600-$850 per square foot. A 400 sqft room addition generally ranges from $240K to $340K. Second-story additions may cost more due to structural reinforcement. Custom Home locks in exact pricing during Phase 1.
How long does a home addition take in Portola Valley?
Home additions in Portola Valley typically take 6-12 months from design to completion. Second-story additions require 8-14 months due to additional structural work. Custom Home's design phase ensures construction runs efficiently and on schedule.
What is the difference between a bump-out and a full addition?
A bump-out extends an existing room by a few feet, while a full addition creates entirely new enclosed space. Bump-outs are smaller in scope and cost, while full additions offer more design flexibility. Custom Home helps you determine the right approach during Phase 1.