Altadena, CA · West San Gabriel Valley
Room addition prices in Altadena, CA: 2026 ranges
A room addition in Altadena costs $325–$650 per sq ft, with a typical mid-size room addition running $119k–$238k. Small projects start around $54k; large or high-end work can reach $432k or more.
As of 2026, costs in Altadena run about 8% above the LA County baseline due to Eaton Fire rebuilds and LA County rebuild permitting.
Updated
Room Addition Cost by Size in Altadena
Totals adjusted for the Altadena market (1.08× LA County baseline). Ranges include labor, materials, permits, and contractor overhead.
| Size | Type | Range | Low | High |
|---|---|---|---|---|
| Small | Bump-out / single room | 150–250 sq ft | $54k | $119k |
| Medium | Suite addition | 300–500 sq ft | $119k | $238k |
| Large | Second story / multi-room | 600+ sq ft | $216k | $432k |
What Affects Room Addition Cost in Altadena?
- Foundation type.
- Roofline integration.
- MEP (mechanical/electrical/plumbing) extension.
- Permits.
- Soil condition.
- Altadena market premium: local pricing reflects Eaton Fire rebuilds and LA County rebuild permitting, which shifts totals about 8% above the LA County baseline.
Get exact room addition bids in Altadena
Two or three licensed builders who work in Altadena, pricing the same scope.
Room Addition Sub-Type Comparisons
Bedroom Addition vs Master Suite Addition
A standard bedroom addition runs $50k–$110k; a master suite (bedroom + bath + walk-in closet) doubles the plumbing scope and lands at $110k–$220k.
Second Story Addition vs Sunroom Addition
Second-story additions require foundation reinforcement and full re-roof at $200k–$400k; sunrooms (prefab or stick-built) skip much of the structural work at $30k–$90k.
How to Get the Best Price on a Room Addition in Altadena
- Get line-itemized bids from two or three builders on the same scope; Altadena room addition pricing commonly varies 10–25% between contractors.
- Lock major materials and finishes before signing; mid-project change orders are the #1 cost driver on room addition jobs.
- Bid in shoulder seasons (Jan–Mar or Sep–Oct) when Altadena contractor schedules are open and rush premiums disappear.
- Verify CSLB license, insurance, and bond before paying any deposit. California caps deposits at $1,000 or 10% of contract, whichever is less.
What is a room addition?
A room addition adds conditioned square footage to a house: a bedroom, a primary suite, a family room or a second story. It needs foundation or framing work, a roof tie-in, and extended electrical, plumbing and HVAC.
How to get an accurate price
- 1.Write down the scope: size, any layout changes, the finish level, and who supplies fixtures and materials.
- 2.Get bids from two or three licensed builders who work in Altadena, all pricing that same room addition scope.
- 3.Line the bids up item by item. Check allowances, permits, exclusions, and whether overhead and profit are shown.
- 4.Before any deposit, check each builder's CSLB license and insurance. As of 2026, California caps a home improvement down payment at $1,000 or 10% of the contract, whichever is less, according to the Contractors State License Board.
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Questions about Room Addition in Altadena
How much does a room addition cost in Altadena, CA?
A room addition in Altadena costs $54k–$432k, with a typical mid-size room addition running $119k–$238k. Per-unit pricing falls in the $325–$650 per sq ft range.
Do I need a permit for a room addition in Altadena?
Yes for most scopes — LADBS or the local city building department requires permits any time you move plumbing, gas, electrical, or alter structure. Like-for-like cosmetic refreshes generally don't.
How long does a room addition take in Altadena?
Plan for 4–14 weeks of construction in Altadena depending on size and complexity, plus 2–8 weeks for design, permits, and material lead times.
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Get Matched with Licensed Altadena Contractors
Tell us about your room addition and we'll line up two or three licensed builders who work in the area.