Additions are where budgets drift the furthest, and almost never because of the things people worry about. Nobody's addition went over because of the tile. It went over because of what was holding the roof up.
Structure is the whole ballgame
The moment you remove a bearing wall, raise a roof or add a storey, you are changing how the building carries load and resists lateral force. That means engineering, new framing, a continuous load path, and often a foundation upgrade underneath the new work.
This is also why two quotes for “a 600 sq ft addition” can differ by a factor of three. One of them has looked at the framing; the other has looked at the square footage.
Where the money actually goes
- ✓Structural engineering and the framing the calculations require
- ✓Foundation work under new or newly loaded walls
- ✓Bringing affected parts of the existing house up to current code
- ✓Re-routing mechanical, plumbing and electrical around the new structure
- ✓Jurisdiction complexity — extra reports, extra sheets, extra review cycles
Second storey versus footprint
Going up preserves your yard but almost always means a foundation upgrade, a new stair, and a temporary roof-off period. Going out is structurally simpler and usually cheaper per square foot, but it consumes lot coverage and can push you into setback or drainage problems. The right answer is a site-specific question, and it is worth deciding it before schematic design rather than during it.
“Decide up or out before schematic design. Changing your mind afterwards is the single most expensive thing a homeowner can do.”
— Devon P., AIA · Licensed CA
Get the structural approach settled early, agree the fee structure in writing with the architect you hire and pay directly, and the rest of an addition is surprisingly predictable.