The honest answer is that two decks with the same footprint can differ in price by a factor of three, and neither quote is wrong. What moves the number is structure more than surface: height off the ground, the slope of the yard, the condition of the ledger and footings, and how the stairs and railings land all cost real framing dollars before a single finish board is chosen.
Materials are the second lever. Pressure-treated framing with a cedar or redwood surface keeps the budget sane and can be refinished for years. Composite decking costs more up front but asks very little back — no sanding, no staining — which pencils out on fog-heavy decks that stay damp for months.
In San Francisco specifically, access is its own line item. A deck reachable only through a narrow side yard or built on a downhill slope means more staging, more hand-carrying, and more engineering. On the Peninsula, the same deck on a flat lot usually lands noticeably lower.
The way to compare quotes is item by item: footings, framing, decking, railings, stairs, permits. At ISMA Builders every estimate is itemized in writing, free, and projects over $1,000 get 5% off — so the number you see is the number you pay.