How much does a 20x20 home addition cost?

HomeGuide publishes $50,000 to $100,000 for a 20x20 room addition — 400 square feet at its $125 to $250 per square foot. Pick 20 x 20 above and the tool shows that published row, and what the same footprint reads as an attic, basement or garage conversion instead.

How much does a second storey addition cost per square foot?

The two sources disagree and the tool prints both. HomeGuide publishes $200 to $500 or more per square foot; HomeAdvisor publishes $100 to $300, rising toward $500 with premium materials, with a normal range of $100,000 to $250,000 and an average of $175,000 for the whole job.

Why does changing the addition type change the number so much?

Because HomeGuide publishes a different band for each one, and the spread is genuine: $50 to $100 per square foot for a basement conversion against $200 to $500 or more for a second storey. A conversion reuses a foundation and a roof. A second storey has to make the existing house carry a floor it was never designed for.

Why is there no region control?

Because neither source publishes a regional multiplier. HomeGuide names zoning, complexity and finishes as things that move a quote without pricing any of them as a factor, and HomeAdvisor says location affects material cost without giving a figure. A slider that scaled your number would be our invention wearing a source's name.

What is the planning and site prep line?

HomeGuide publishes six lines that come before the build: house plans, a land survey, site excavation, grading, the foundation and building permits. It publishes the six and no total, so the optional line in the tool is our addition of its six published ranges and is labelled that way wherever it appears.

Does this calculator give me a quote?

No. It gives you the published range for your footprint and addition type and shows what each line does to it. Only a contractor who has walked the house can quote the job, and you should get three of those in writing for the same square footage and the same finish level.

Home addition cost calculator

A home addition costs $125 to $250 per square foot, or $50,000 to $100,000for a 20x20 room addition, on HomeGuide’s published figures. This calculator reads your footprint and addition type off the same tables, with every figure sourced.

This page is general information, not professional advice. It cannot see your house’s foundation, its roof line or what your building department will ask for, and the only number that counts is a written quote from a contractor who has walked the job.

1. How big is the addition?

Pick one of the footprints HomeGuidepublishes a price for, or enter your own square footage and the tool applies HomeGuide’s published per-square-foot band — the unit it prices additions in.

2. Which kind of addition?

This is the control that moves the number most. HomeGuide publishes a separate per-square-foot band for each type, from $50 for an attic or basement conversion to $200 and up for a second storey.

Room addition — $125 to $250 per sq ftA completely new room built onto the house, on its own foundation. HomeGuide's by-footprint table is published for this type.Bump-out extension — $85 to $200 per sq ftPushing an existing room out, sometimes only a couple of feet. HomeGuide says cantilevering it off the existing foundation, with no new footings, is what saves the money.ADU addition — $100 to $300 per sq ftAn in-law suite built for full-time living, with its own bedroom, kitchen and bathroom.Attic conversion — $50 to $150 per sq ftFinishing space that already has a roof over it. HomeGuide says converting to a primary suite with an en suite costs more than a guest bedroom.Basement conversion — $50 to $100 per sq ftTurning an unfinished basement into livable space. HomeGuide's cheaper $30 to $50 band is for finishing alone rather than a full home addition.Garage conversion — $50 to $200 per sq ftRepurposing the garage. HomeGuide says a flex space without plumbing is the cheaper and faster version.Second-storey addition — $200 to $500+ per sq ftThe most expensive type on HomeGuide's table, because the foundation, walls and roof all have to be modified to carry the weight. HomeAdvisor prices the same job lower — both bands are shown.Sunroom addition — $100 to $350 per sq ftHomeGuide's band spans three-season and four-season rooms; the four-season version has insulation, electricity and heating and cooling.

3. Add the planning and site prep?

HomeGuide prices six lines that come before the build — plans, survey, excavation, grading, foundation and permits. It publishes the six lines and no total, so the total below is our addition of its six published ranges, and it is labelled that way everywhere it appears.

20 x 20 ft room addition

Published estimate

$50,000 – $100,000

HomeGuide’s published row for a 20 x 20 ft room addition, plus the planning line if you included it. Not a quote — the spread is real, and your written quotes will land somewhere inside it.

The published row
$50,000 to $100,000 for a 20 x 20 ft room addition on HomeGuide's footprint table — which is its $125 to $250 rate across 400 square feet.
Planning and site prep — left out
Not in the total. HomeGuide publishes six lines that come before the build and they add to $7,000 to $29,300 on our arithmetic. A conversion may skip the excavation and the foundation; a new room will not.

Cheaper shapes of the same idea

HomeGuide prices a dormer at $19,600 to $54,900 for 25 to 250 square feet without touching the existing roofline, a bump-out or cantilever at $10,000 to $30,000, and a second storey over an existing garage at $60,000 to $200,000+. Each is a different job from a full addition, which is why none of them is a control on this tool.

Every choice above is in the web address, so the link opens on the same numbers.

There is no region control here, and no finish-quality slider, because neither source publishes one. HomeGuide names size, complexity, zoning, features, interior finishes and permit delays as the things that move a real quote, and prices none of them as a multiplier; HomeAdvisorsays location affects material cost without giving a figure. A slider that scaled your number would be our invention wearing a source’s name, so they are printed as questions to ask a bidder instead.

HomeGuide’s published home addition cost by footprint. The table is published for a room addition.
Footprint	Square feet	Room addition
10 x 10 ft	100	$12,500 – $25,000
12 x 12 ft	144	$18,000 – $36,000
10 x 20 ft	200	$25,000 – $50,000
20 x 20 ft	400	$50,000 – $100,000
20 x 30 ft	600	$75,000 – $150,000
15 x 40 ft	600	$75,000 – $150,000
20 x 40 ft	800	$100,000 – $200,000
25 x 40 ft	1,000	$125,000 – $250,000

Source: HomeGuide.

HomeGuide’s published home addition cost by room. These are its own totals for its own size ranges — a readout, never multiplied by anything on this page.
Room	Typical size	Published total
Bathroom	40 – 50 sq ft	$7,000 – $11,000
Bedroom	120 – 140 sq ft	$15,000 – $25,000
Family or flex room	200 – 225 sq ft	$25,000 – $40,000
Kitchen	100 – 200 sq ft	$20,000 – $50,000
Primary suite	300 – 600 sq ft	$45,000 – $135,000
Sunroom	150 – 300 sq ft	$15,000 – $105,000

Source: HomeGuide.

HomeGuide’s published planning and site-prep lines. The optional line in the tool is our addition of these six ranges; HomeGuide publishes no total of its own.
Line	Published cost	What it covers
House plans	$1,200 – $5,000	Drafted by an engineer or an architect.
Land survey	$400 – $1,800	Marks foundations, utilities, boundaries and topography.
Site excavation	$1,500 – $5,000	Digging the foundation pad for the extension.
Grading and levelling	$1,000 – $5,000	Levelling the yard before the foundation goes in.
Foundation	$2,400 – $10,500	Pouring a concrete slab foundation for the addition.
Building permits	$500 – $2,000	Depends on local permitting requirements and fees.

Source: HomeGuide. Our sum of the six: $7,000 to $29,300.

Figures last read from their sources on August 22, 2026.

Where every price on this page comes from

HomeGuide supplies the footprint table, the per-square-foot band, the eight addition types, the by-room totals and the six planning and site-prep lines. HomeGuide supplies the second-storey band and the dormer, bump-out and over-garage comparisons. HomeAdvisor is the second reading on the second storey. Each was read first-hand on the date stamped above, and each is linked next to the figure it produced.

Where the sources disagree, both are printed rather than smoothed over. HomeGuide prices a second storey at $200 to $500 or more per square foot; HomeAdvisor prices the same job at $100 to $300, rising toward $500 with premium materials. That is a real gap on a real question, and averaging the two would invent a number neither publisher stands behind.

Why the footprint table only drives a room addition

Because that is the job HomeGuide published it for. Its footprint table sits directly under its $125 to $250 room-addition rate, and every row on it is that rate times its own area. Reading the $50,000 to $100,000row as the price of a 400-square-foot attic conversion would put HomeGuide’s name on a figure it never published, so the tool switches to HomeGuide’s own band for the type you picked and shows the room-addition row beside it, labelled as what it is.

The one number here that is ours

The planning and site-prep line. HomeGuide publishes six ranges — house plans, land survey, site excavation, grading and levelling, foundation and building permits — and publishes no total of them. Adding them gives $7,000 to $29,300, and that is our arithmetic on its published figures rather than a figure it stands behind. It is optional, it is itemised, and it says so every time it renders. A conversion that reuses an existing foundation will skip two of the six lines; a new room on a new slab will not.

The deeper pages behind this tool: home addition cost by footprint, what a second storey costs and what it costs to build an ADU.

Questions people ask about this

How much does a 20x20 home addition cost?: HomeGuide publishes $50,000 to $100,000 for a 20x20 room addition — 400 square feet at its $125 to $250 per square foot. Pick 20 x 20 above and the tool shows that published row, and what the same footprint reads as an attic, basement or garage conversion instead.
How much does a second storey addition cost per square foot?: The two sources disagree and the tool prints both. HomeGuide publishes $200 to $500 or more per square foot; HomeAdvisor publishes $100 to $300, rising toward $500 with premium materials, with a normal range of $100,000 to $250,000 and an average of $175,000 for the whole job.
Why does changing the addition type change the number so much?: Because HomeGuide publishes a different band for each one, and the spread is genuine: $50 to $100 per square foot for a basement conversion against $200 to $500 or more for a second storey. A conversion reuses a foundation and a roof. A second storey has to make the existing house carry a floor it was never designed for.
Why is there no region control?: Because neither source publishes a regional multiplier. HomeGuide names zoning, complexity and finishes as things that move a quote without pricing any of them as a factor, and HomeAdvisor says location affects material cost without giving a figure. A slider that scaled your number would be our invention wearing a source's name.
What is the planning and site prep line?: HomeGuide publishes six lines that come before the build: house plans, a land survey, site excavation, grading, the foundation and building permits. It publishes the six and no total, so the optional line in the tool is our addition of its six published ranges and is labelled that way wherever it appears.
Does this calculator give me a quote?: No. It gives you the published range for your footprint and addition type and shows what each line does to it. Only a contractor who has walked the house can quote the job, and you should get three of those in writing for the same square footage and the same finish level.

When you are ready for those three written quotes, these are the general contractors AI assistants actually name when homeowners ask. Nobody can pay to be on these lists.