How to read an addition budget
Measure the outside of what you plan to add, then choose its type. The rates come from two kinds of source, and they disagree. Contractor guides such as Block Renovation give rates per square foot, and call them planning guardrails. Cost vs. Value prices one fixed job in full, with its size and contents defined, as a national average. Those fixed jobs often work out to far more.
So a range here is wide on purpose. For a bathroom, it runs from Block's low end to the rate of Cost vs. Value's small midrange bath, which is about $1,263 per square foot. A small job costs a lot per foot, as Block says, because fixed costs do not shrink. If your addition is under 100 square feet, plan near the top of the range. The finish level picks part of that range and the region adjusts it, and both of those steps are our own model.
We could read no source that lists what additions usually leave out. On this site, we treat structural work, permits and anything found behind a wall as not included, and design fees and lot setback limits as things to ask about. Cost vs. Value's jobs are built over a crawlspace, so a slab or a basement can change them.
A worked example
Here is a 20 by 15 foot bedroom at a standard finish, at the baseline region. These lines come from the same formula the calculator above runs.
20 by 15 ft bedroom, standard finish
- 20 × 15 = 300 sq ft
- Low end: $45,000
- High end: $105,000
- Middle: $75,000
The same room in the Northeast comes to $49,500 to $115,500 on the site's regional model. For work inside the walls you have, try the basement finishing calculator, which may reach the same goal for less, or the home renovation calculator for a whole-house budget.
What the sources print
The rate column is our own division. Notice how small jobs cost more per foot, and how the two kinds of source differ. Cost vs. Value's latest edition we could read was its 2025 report.
| Job | Size | Cost | Works out to per sq ft |
|---|---|---|---|
| Bathroom, midrange, Cost vs. Value | 48 sq ft | $60,645 | $1,263 |
| Bathroom, upscale, Cost vs. Value | 100 sq ft | $111,255 | $1,113 |
| Primary suite, midrange, Cost vs. Value | 384 sq ft | $170,517 | $444 |
| Primary suite, upscale, Cost vs. Value | 640 sq ft | $351,613 | $549 |
| Primary suite, Block Renovation | 600 sq ft | $150,000 to $300,000 or more | $250 to $500 |
Mistakes when budgeting an addition
- One flat rate on a small job. A flat rate underprices small additions, since fixed costs do not shrink.
- Comparing unlike figures. Cost vs. Value defines its job, while the guides do not say what they include.
- Assuming upward is cheaper. Sources split on whether a second story saves money, so have an engineer look first.
- Forgetting the lot. Setback limits may rule out the size you drew, so check before you pay for plans.
- Trusting our regional factor too far. It is the site's own model, and it is broad. Local bids settle it.
Questions
How much does a room addition cost per square foot?
About $150 to $350 for a basic room, by Block Renovation, and that firm calls these guardrails and not bids. Fixed jobs in Cost vs. Value run far higher: about $444 per square foot for a midrange primary suite with its bath and closet.
What is the cost to add a bedroom to a house?
About $45,000 to $105,000 for a 20 by 15 foot room, at Block's $150 to $350 per square foot. A closet, heating and a larger foundation can push it up. Ask for a bid that shows each of these as its own line.
What is the cost to add a bedroom and bathroom to a house?
Run each part here and add them, or use the suite option. Block gives $150,000 to $300,000 or more for a 600 square foot primary suite. Cost vs. Value lists $170,517 for a 384 square foot suite with a bath and closet, on a crawlspace.
How much does a bump-out addition cost per square foot?
About $85 to $200 on a contractor-site figure we read only as a search excerpt. Block Renovation warns that small jobs can still run high per square foot, because fixed costs do not shrink, so treat this as the least certain rate on the page.
What does a 20x20 room addition cost?
About $60,000 to $140,000 for a basic room at 400 square feet, on Block's rates. A midrange finish with its own bath moves it far higher, and Cost vs. Value's suite works out to about $444 per square foot. Get a local bid.
Is a second-story addition more expensive?
Not always, as the sources split, and we read those only as search excerpts. One says building up avoids foundation and digging. Another says it adds structural work and a new roof. Block's range is $200 to $500 per square foot, so ask an engineer first.
What is the cost to add a bedroom above a garage?
We found no source for this job on its own, so the second story option is the nearest fit, at about $200 to $500 per square foot. The garage's walls and footings may need work, which an engineer can check before you price it.
This room addition cost is a planning range, and it stretches far because contractor guides and Cost vs. Value differ by several times. Structural work, design fees and zoning rules can move it too. A surprise behind an old wall is common, and permits vary. Only a written bid from a licensed contractor who has seen your house means anything firm.
Sources and review
- The home addition costs guide, Block Renovation, 2026.
- Cost vs. Value, primary suite addition, midrange, Zonda, via JLC, 2025.
- Cost vs. Value, primary suite addition, upscale, Zonda, via JLC, 2025.
- Cost vs. Value, bathroom addition, midrange, Zonda, via JLC, 2025.
- Cost vs. Value, bathroom addition, upscale, Zonda, via JLC, 2025.
- How much does a bump-out addition cost?, HomeGuide, 2026, read only as a search excerpt.
Checked in November 2026. Spotted a wrong number? Report it and it goes in the corrections log.
How this is calculated
Every figure below is read from the code that produces the answer, so what is written here is what the calculator ran. The constants were last checked against their sources in November 2026.
The arithmetic
Range
Range = Area × Rate × Regional factor
- Rate
- Dollars per square foot for the type and finish
Worked through: 300 × 150 × 1 = 45000
The constants, and where they come from
COST_PER_SQ_FT
Modelled from published dataDollars per square foot by type of addition, low and high
- bedroom
- 150, 350
- bathroom
- 250, 1263
- primary-suite
- 250, 549
- second-story
- 200, 500
- bump-out
- 85, 200
Applies to: United States, from a contractor's guide and a national average of fixed jobs
Source: The home addition costs guide, Block Renovation, published 2026-07, read 2026-11.
Also from: Cost vs. Value, bathroom addition, midrange, Zonda, via JLC, published 2025-06, read 2026-11.
Also from: Cost vs. Value, bathroom addition, upscale, Zonda, via JLC, published 2025-06, read 2026-11.
Also from: Cost vs. Value, primary suite addition, midrange, Zonda, via JLC, published 2025-06, read 2026-11.
Also from: Cost vs. Value, primary suite addition, upscale, Zonda, via JLC, published 2025-06, read 2026-11.
Also from: How much does a bump-out addition cost?, HomeGuide, published 2026, read 2026-11.
Block gives the low ends and the bedroom and second story ranges. Cost vs. Value sets the top for a bath and a suite. The bump-out range comes from a HomeGuide figure read only as an excerpt, and it is the least certain.
SMALL_JOB_SQ_FT
Modelled from published dataSize under which a warning about fixed costs is shown, in square feet
Applies to: Our own step, based on a general point
Source: The home addition costs guide, Block Renovation, published 2026-07, read 2026-11.
Also from: Cost vs. Value, bathroom addition, upscale, Zonda, via JLC, published 2025-06, read 2026-11.
Block says fixed costs do not shrink on a small job, and gives no size. We chose 100 square feet from Cost vs. Value's bath, which is over $1,000 per square foot at that size.
PROJECT_MULTIPLIER
ModelledMultiplier by United States region, applied to this page's modelled rates
- northeast
- 1.1
- southeast
- 0.94
- midwest
- 0.97
- southwest
- 1
- west
- 1.13
Applies to: United States, five broad regions
No published figure exists for this. It is derived, and the derivation is below rather than a citation to something that does not say it. Last checked in November 2026.
The site's own model, shared across cost calculators. No published series was found, and the small-site regional figures we saw were not reliable enough to use.
What the labels mean
- Modelled from published data:
- derived rather than measured, calibrated against the source named beside it.
- Modelled:
- derived rather than measured, and the derivation is shown.
If a figure here looks wrong, it may be. Tell us what you found and we will check it against the source and publish a correction if it needs one. How constants are chosen and how often they are reviewed is set out in our methodology.
How to use the room addition cost calculator
- 1
Enter the size
- 2
Choose the type of addition
- 3
Choose a finish level
- 4
Choose your region
- 5
Take the range to a licensed contractor, and ask for an itemized bid
What this calculator assumes
Every estimate here rests on the assumptions below. Where your job differs from one of them, the figure will differ too.
- Rates come from contractor guides, with Cost vs. Value at the top end for a bathroom and a suite
- A finish level picks part of the range, which is our own model
- The regional multiplier is the site's own model
- Permits, design fees and structural work are not separately added
Pro tips
- Ask each bidder for the foundation, the shell and the finishes as separate lines
- Check your lot's setback limits before you draw anything
- Have an engineer look before you plan to build upward