Estimate Desk
Home addition cost estimate
A room on the side of the house. Foundation and roof tie-in are inside the square-foot rate. The band is a Kitsap planning range, about ±15%, not a contract price.
Default scenario
240 sf, one story, standard finish
$105,000 – $142,000
ADUs, additions, and basement suites. Drawing and permit lines are called out, not buried.
Planning band
$105,000 – $142,000
About 35 working days on site. Not a bid.
Line items, notes and what is not in the band are below the form.
Planning bandLine items and notes
$105,000 – $142,000
Point total about $123,000 · about 35 working days on site
Material and labor of the unit-priced lines: Material $52,200 · Labor $52,200
Default scenario: 240 sf, one story, standard finish. Not a contract price. The band is the point total plus or minus 15%, a planning allowance we set for early estimates, not a measured accuracy. Prices include materials, labor, JKM overhead and fee, and 9.3% Washington sales tax. Working days on site are crew days. Design, permits and material lead times add calendar time before and between them.
| Item | Qty | Total |
|---|---|---|
| Addition — foundation, frame, dry-in, and finish | 240 sf | $104,400 |
| Design and permit drawings | 1 ls | $9,800 |
| Permit fee allowance | 1 ls | $2,200 |
| Contingency (6%) | 1 ls | $6,984 |
Notes
- The rate assumes the addition lands on conventional soil and ties into an existing roof of ordinary pitch.
Not in this band
- Hazardous-material abatement. Asbestos and lead are common on pre-1978 houses and are separately licensed work.
- Structural repair discovered after demolition.
- A new electrical panel, water service, or septic, unless that line is on this estimate.
- Design fees, except where a drawing line is shown.
- Financing and temporary housing. (Washington sales tax, 9.3%, is included in these prices.)
- A new septic or a panel upgrade, unless electrical calculator is added on.
Planning band for Kitsap County. Not a bid. A site visit sets the number.
What the addition estimate covers
The addition estimate prices a room or rooms built onto an existing house, from the floor area, the number of stories and the finish level. The square-foot rate covers the foundation, the frame, the dry-in and the finish, including the roof tie-in. Design and permit drawings, and an allowance for the permit fee, are lines of their own, and a switch adds a bath inside the addition.
The default is a one-story addition of modest size at standard finish, on a house built in 1978 or later.
What moves the number
Area is the base, and finish level moves the rate in three steps: builder, standard and premium. A second story raises the rate because the work is slower and needs more structure. The bath is a fixed-price line that appears when you switch it on, and it also changes the plumbing and the schedule.
The drawing allowance steps with the size of the addition, since a very small addition needs less design than a larger one. Days on site follow the area.
Contingency and the age of the house
An addition opens up an existing house, so the desk holds a contingency inside the point total. It is a smaller percentage when the house was built in 1978 or later and a larger one when it was built before 1978, where wiring, moisture and finishes tend to hold surprises. The percentages are shown on the how estimates work page.
What is not in the band
The rate assumes conventional soil and an existing roof of ordinary pitch. A new septic or a panel upgrade is not in the band unless you add the electrical estimate on top. The usual exclusions also apply: hazardous-material abatement, structural repair found once the house is opened, design fees beyond the drawing line, financing and temporary housing. Washington sales tax is already included in the prices.
An addition needs a permit, and the office that issues it depends on where the house sits. Our permit guides explain how the five Kitsap authorities divide the communities, and the estimate carries a permit allowance so the line is visible. Use the band to see whether the project fits your budget, then ask for a site visit so we can look at the soil, the roof and the existing structure before we write the real estimate.
Scope ladder
Addition: three scopes compared
Each row is the Estimate Desk engine run on the inputs listed, so these numbers move whenever the desk does. They are planning bands, about ±15%, not bids.
| Scope and what was priced | Planning band |
|---|---|
Builder finishA one-story addition at builder finish, with no bath, on a house built in 1978 or later.
| $93,000 – $126,000Point total about $109,000About 35 working days |
Standard finishDesk defaultA one-story addition at standard finish, with no bath, on a house built in 1978 or later.
| $105,000 – $142,000Point total about $123,000About 35 working days |
Larger, premium, with a bathA larger one-story addition at premium finish that includes a bath, on a house built in 1978 or later.
| $238,000 – $322,000Point total about $280,000About 48 working days |
Questions
Addition estimates: common questions
What does the addition estimate assume before I change anything?
The starting band, $105,000 – $142,000, is the default scenario (240 sf, one story, standard finish). It prices the foundation, frame, dry-in and finish by the square foot, design and permit drawings, a permit fee allowance and a contingency inside the point total.
What is included in the square-foot rate for an addition?
The foundation, the frame, the dry-in and the finish. The roof tie-in is part of it as well. Design and permit drawings and the permit fee are separate lines, because they are real costs that arrive before the build.
Why does the addition estimate hold a contingency?
Because an addition ties into an existing house and what is behind its walls is unknown. The contingency sits inside the point total and is larger when the house was built before 1978.
What assumptions does the addition estimate make about the site?
That the addition lands on conventional soil and ties into an existing roof of ordinary pitch. Poor soil, a steep or unusual roof, a new septic or a panel upgrade are outside the band.
A site visit sets the number
Bring your result to the walk. We measure, check what is behind the walls, and replace the planning band with a written estimate.