Why does room addition cost vary by city?

Local labor rates, permit timelines, and demand differ across Southern California. Each city in our guides applies its own cost multiplier to the regional baseline, so a room addition in a coastal premium market costs more than the same scope inland.

Is the price worth it — and how do I avoid overpaying?

Get at least three written bids for the identical scope, make finishes allowance-based, and check that permits and overhead-and-profit are shown as their own lines. Keystone matches you with up to 3 vetted contractors and will review a bid you already have.

Room Addition Cost in Southern California (2026)

Q: How much does a room addition cost in Southern California in 2026?

A mid-size room addition typically runs $110k–$220k at the LA County baseline. Small projects start around $50k; large or high-end work can reach $400k or more. Orange County runs slightly above this and San Bernardino County below.

A mid-size room addition typically runs $110k–$220k at the LA County baseline ($300–$600 per sq ft). Small projects start around $50k; large or high-end work can reach $400k or more.

Pricing varies across Southern California — Orange County runs a bit above the LA baseline and San Bernardino County below. Compare by county and city below.

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Room Addition Cost by Size

Southern California baseline ranges (LA County = 1.0× baseline). Individual counties and cities vary — see below.

Size	Type	Range	Low	High
Small	Bump-out / single room	150–250 sq ft	$50k	$110k
Medium	Suite addition	300–500 sq ft	$110k	$220k
Large	Second story / multi-room	600+ sq ft	$200k	$400k

Room Addition Cost by County

Los Angeles County

$110k–$220k

mid-size · 1.00× baseline

See LA County city-by-city

Orange County

$116k–$231k

mid-size · 1.05× baseline

See OC city-by-city

San Bernardino County

$94k–$187k

mid-size · 0.85× baseline

See SB County city-by-city

Room Addition Cost by City

What Affects Room Addition Cost?

Foundation type
Roofline integration
MEP (mechanical/electrical/plumbing) extension
Permits
Soil condition

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Room Addition Cost FAQs

Keep exploring

Compare other project costs, drill into room addition by area, or read our guides.

Other project cost guides

Room Addition by area

Renovation guides

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