Additions · Cost
What does a home addition cost?
The square footage is the easy part to price. What's underneath it is where additions go over budget.
Home additions typically run $130,000 to $490,000, driven by how much square footage you're adding and how much of the existing structure has to change to carry it.
What you're actually paying for
- Foundation. New footings, engineered to your soils report, tied into what's already there.
- Structural changes to the existing house. Opening a wall to connect the addition means something else has to carry that load. This is where engineering costs live.
- Framing and roof. Tying a new roof into an existing one properly is more work than framing new construction from scratch.
- Systems. Whether your panel, water heater and HVAC can carry the added load, or whether they become part of the project.
- Finishes. Real money, but the most controllable line on the job.
The part that most often goes wrong
It is almost always the foundation, and specifically what is found once demolition starts. Additions connect to existing construction, and existing construction is full of things nobody documented.
On a Milpitas addition the approved plans called for building on an existing concrete slab. Cutting into it to dig the piers showed it was an unengineered slab poured over an old patio — sand between the pours, effectively no reinforcement. The whole thing came out, the pad was rebuilt to the geotechnical spec, and it went through as a written change order before any work continued.
See the photos of what was under that slab →
This is the reason we ask about the foundation before pricing, and the reason a responsible bid on an older home carries contingency for it. A bid that assumes everything below grade is fine is not cheaper — it is just less finished.
Addition or ADU?
They solve different problems. An addition grows the house you live in and is usually the answer when you need more connected living space. An ADU is a separate legal dwelling with its own entrance, which is the answer if you want rental income or genuinely independent space for family.
On cost, they are closer than people expect. An ADU carries about $13,500–$18,500 of soft costs an addition often doesn't, but skips the structural surgery on your existing house. Which is cheaper depends on your lot and your house, not on a rule.
How we price it
By trade, with allowances shown as allowances rather than buried in a padded fixed price. Anything that genuinely cannot be known until something is opened up is called out as an allowance and billed at actual cost, with whatever is unspent credited back. How payments work →
Common questions
How much does a home addition cost in San Jose?
What makes a home addition go over budget?
Is an addition cheaper than an ADU?
Do I need an engineer for a home addition?
Where to go next
Written by the team at Elusive Construction, a licensed California general contractor (B #1112902) building in Santa Clara and San Mateo counties. General information about how residential construction works — not legal advice. For your specific contract, talk to an attorney.
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