Lamont Bros. Home Addition Cost Calculator

Design-build pricing, calibrated to our real Portland-area additions

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Running estimate $0 —

Design-Build · Portland, OR

Estimate the cost of your home addition.

Tell us the size and scope of the addition you're planning — we'll generate a custom estimate you can use to move your project forward.

Takes about 3 minutes No email required Built from real Portland project data

What kind of addition are you planning?

Additions range from small bump-outs to full second stories and ADUs. Pick the option closest to what you have in mind — we'll refine details in the steps ahead.

Bump-out

Small single-room expansion, ~100 sq ft

Extending an existing room by a small footprint — often to expand a kitchen for better cooking space, add a nook or breakfast area, or grow a bathroom. A Chef's Kitchen (right) was a 100 sq ft kitchen bump-out.

$50,000 – $120,000

See an example project →

Single-Story Addition

Full new room on existing floor

A standalone footprint added to your ground floor — often a family room, primary suite, or larger kitchen. Includes new foundation, framing, roof integration, and interior finish.

$120,000 – $250,000

See an example project →

Second-Story or Over-Garage

New full floor added above

Adding a second story or building above an attached garage. Includes structural reinforcement of the existing home, roofline changes, new stairs, and full interior build-out.

$250,000 – $450,000

See an example project →

ADU or Suite Addition

Full living space with kitchen + bath

A complete accessory dwelling unit or in-law suite — self-contained living with its own kitchen, bathroom, and often separate entry. May be attached or detached.

$300,000 – $500,000+

See an example project →

How to price out your addition

Home additions represent one of the largest investments a homeowner can make in their home. Lamont Bros. divides additions into three categories: Simple ($150k-$250k), Standard ($300k-$500k), and Complex ($500k-$800k+) — with the average Portland addition running about $469 per square foot.

Bigger additions can actually cost less per square foot than smaller ones, because fixed costs (design, permits, mobilization) get spread across more square footage. That's why a 300 sqft addition may come in at $802/sqft while a 998 sqft two-story lands at $348/sqft.

Read the full home addition cost guide →

Second-Story Hillsboro Second Story Addition
Hillsboro, OR

Second-Story Sweet Green Second Story Addition
Portland metro

Single-Story Damascus Single-Level Addition
Damascus, OR

ADU / Suite Woodlawn ADU
Woodlawn · Portland, OR

Garage Conversion Portland Garage Conversion
Portland, OR

Kitchen Addition Garden Home Kitchen Addition
Garden Home · Kitchen expansion

What kind of foundation does the addition need?

Foundation type is often the largest single cost driver of an addition. Bump-outs may not need one at all; full second stories need to inherit the load of the existing foundation.

Cantilever (no new foundation)

Small bump-outs only

The new floor cantilevers off existing framing. Feasible for small bumps up to 2–3 feet, no ground-level foundation work needed.

$0 / sqft

Extend existing crawl space

Most common for older Portland homes

Adds new footings and crawl space walls tied to your existing crawl space. Most common approach for single-story additions on Portland's Craftsman and Foursquare homes.

~$50 / sqft

New slab foundation

Common for garages, ADUs, sunrooms

A concrete slab on grade. Typical for ADUs, garage conversions, and additions where you don't need a crawl space or basement below.

~$75 / sqft

Full basement foundation

Adds usable space below

Excavation for a full basement below the addition. Doubles your usable square footage but is the biggest single line item on the estimate.

~$150 / sqft

Can I add an addition over my garage?

Foundation type is often the biggest single line item on an addition estimate. Bump-outs can cantilever off existing framing with no new foundation. Single-story additions typically extend the existing crawl space. ADUs and garages often use a new slab. Second-story additions inherit the existing foundation but require structural reinforcement to carry the added load.

Adding a story over an existing garage is a common way to add square footage without expanding the home's footprint — but it requires careful structural analysis of the existing garage foundation and walls.

Read: Can I do an addition over my garage? →

Slab / conversion Portland Garage Conversion
Existing slab reused

Existing foundation Hillsboro Second Story Addition
Inherits existing footprint

Extended crawl space Damascus Single-Level Addition
Crawl space extension

Estimate the cost of your home addition.

What kind of addition are you planning?

How large is the addition?

What kind of foundation does the addition need?

What's the addition for?

What level of interior finish do you want?

Any special systems to include?

Any add-ons to price in?

Your estimated home addition investment

Next step: let's talk about your addition

Where your money goes

Why this range

Meet Lamont Bros.

Your selections