Siding is quoted per square foot or per square, and the material is typically the smaller share of it.
What is inside the price
Tear-off and disposal of the existing siding, where it is being removed. Skipping this by siding over the old material saves money and buries whatever is wrong underneath.
Repair of the sheathing, which cannot be quantified until the old siding is off. This is the contingency item.
Weather-resistive barrier and flashing.
The siding material and trim.
Labour, which is the largest single line on most jobs.
Scaffolding or lifts, on anything above a single storey.
The five things that move it most
House shape. Corners, dormers, bump-outs, gables and returns all mean cuts, trim and slow work. A simple two-storey box costs far less per square foot than a house of the same area with an articulated elevation.
Height. Two storeys means staging, and three means real access equipment. Labour rates effectively rise with height.
Openings. Every window and door needs trim, flashing and cutting. Fifteen windows costs meaningfully more than eight on the same wall area.
Tear-off, and what is found. Rotten sheathing, missing or reverse-lapped wrap, insect damage, or an unexpected second layer of old siding underneath.
Material tier, which is where most people focus and which matters less than the four above.
Where the surprises live
The wall is opened once. What is behind it is unknown until then, and on a house of any age something usually is.
Rotten sheathing, most often below windows and at the bottom of walls.
No flashing at openings, which is common on older houses and is a repair rather than an option.
A second layer of siding, which doubles the tear-off.
Asbestos, in cement siding on houses of a certain era, which requires specialist removal and is a legal requirement rather than a choice.
Lead paint, on anything painted before 1978, which requires certified lead-safe practices.
A contingency of 10 to 20% is appropriate, and the higher end on an older house.
Siding over the top
Installing new siding over the old avoids tear-off and disposal, which is a real saving.
It also seals whatever is underneath — including any rot, any missing flashing, and any moisture problem — permanently inside the wall, and it adds thickness that pushes the new siding out past the window trim, which frequently looks wrong.
It is a legitimate choice on a sound wall with good existing flashing. It is a false economy on a wall where nobody has looked.
Comparing quotes
Ask each contractor for the same four things: whether tear-off is included, what the WRB and flashing specification is, how sheathing repair is priced, and what the material and trim are by name.
Quotes that differ hugely usually differ on tear-off and on what happens when rot is found. Both are worth pinning down before comparing totals.
The item worth paying for
Flashing and the weather barrier. They are a small fraction of the cost and they decide whether the wall behind the new siding stays sound for thirty years.
A contractor who itemises them is thinking about the wall. One who does not mention them is thinking about the siding.
Season and scheduling
Siding is exterior work and it is weather-dependent. Crews are busiest in the warm months and lead times are longest then; a job booked for autumn often starts sooner.
Vinyl in particular is installed differently in cold weather — the expansion gaps are cut larger because the material is contracted — and a crew that does not adjust for temperature produces panels that buckle the following summer.
Payment and holdback
A deposit and progress payments are normal. Hold a meaningful final payment until the job is complete, including the trim details and the clean-up, because the small items at the end are what a siding job is judged on from the street.