Home Addition Cost Guide: Adding a Room or Second Story in 2026

When your home runs out of space, you have two choices: move or build more. If you love your location, a home addition is often the smarter play—but it's also one of the most complex and expensive renovation projects you can take on.

A home addition typically costs $80 to $300 per square foot, putting a standard 400-square-foot room addition in the $32,000 to $120,000 range. A full second-story addition over an existing footprint runs $100 to $350 per square foot, or $100,000 to $500,000+ for a typical project.

Those numbers depend heavily on what you're building, where you live, and what's already underneath your house. Here's how to figure out what your project will actually cost.

Types of Home Additions and Their Costs

Conventional Room Addition: $80–$250 per sq ft

This is the most common type—building out from the existing footprint on the ground floor. You're pouring a new foundation, framing walls, tying into the existing roof, and extending electrical, plumbing, and HVAC systems.

A typical 12x20-foot room addition (240 sq ft) costs $20,000–$60,000 depending on the room type and finish level. A bedroom with a closet is on the low end. A bedroom with an attached bathroom pushes costs higher because of plumbing.

Second-Story Addition: $100–$350 per sq ft

Adding a second story is structurally complex. The existing foundation and first-floor walls may need reinforcement to carry the additional load. You're essentially building a new house on top of your existing one.

A 1,000-square-foot second story over a ranch-style home typically costs $150,000–$350,000, including:

Structural engineering and reinforcement: $10,000–$30,000
Framing and roofing: $40,000–$80,000
Electrical, plumbing, HVAC extension: $20,000–$50,000
Interior finish (drywall, flooring, paint, trim): $30,000–$60,000
Staircase: $3,000–$10,000
Windows and exterior finish: $15,000–$30,000

The advantage of a second story is that you don't sacrifice yard space and you don't need a new foundation (though the existing one may need work).

Bump-Out Addition: $50–$200 per sq ft

A bump-out extends an existing room by 2 to 10 feet. It's smaller and simpler than a full room addition—often you can cantilever the structure without a new foundation (up to about 3 feet). Common uses: expanding a kitchen, adding a breakfast nook, enlarging a bathroom.

A 4x10-foot kitchen bump-out (40 sq ft) typically costs $5,000–$15,000 for a basic cantilevered design or $15,000–$30,000 if it requires a new foundation.

Sunroom or Four-Season Room: $20,000–$80,000

Sunrooms range from prefabricated three-season enclosures ($10,000–$25,000) to fully insulated four-season rooms with HVAC ($30,000–$80,000). The cost depends heavily on whether you need a concrete foundation and full climate control.

In-Law Suite or ADU: $50,000–$200,000+

An accessory dwelling unit (ADU) is essentially a small self-contained apartment—bedroom, bathroom, kitchenette, and separate entrance. Detached ADUs (backyard cottages) cost more than attached ones due to separate utility connections.

Attached in-law suite: $50,000–$120,000 Detached ADU (500–800 sq ft): $100,000–$250,000

Many cities have relaxed ADU zoning requirements in recent years, making this a popular option for multigenerational living or rental income.

Garage Conversion: $20,000–$60,000

Converting an existing attached garage to living space is one of the cheapest ways to add a room because the structure already exists. You're adding insulation, drywall, flooring, windows, and HVAC. The catch: you lose your garage, which can reduce property value in some markets.

What Drives the Cost of a Home Addition?

Foundation

Every ground-level addition needs a foundation. Options include:

Concrete slab: $5–$8 per sq ft — cheapest, works in warm climates
Crawl space: $7–$14 per sq ft — provides access to utilities
Full basement: $20–$40 per sq ft — most expensive but adds usable space

The foundation alone can account for 10%–15% of the total project cost.

Roofing and Exterior

Tying a new roof into an existing one is tricky. A simple shed roof is cheapest. Matching the existing roofline with valleys and ridges costs more. Budget $10,000–$25,000 for roofing on a typical addition. Siding to match the existing exterior adds $3,000–$10,000.

Plumbing

Adding a bathroom to your addition is the single biggest cost escalator. Rough-in plumbing for a new bathroom costs $3,000–$8,000. If your main sewer or water lines need upgrading to handle the additional capacity, add $2,000–$5,000.

Electrical

A room addition needs new circuits, outlets, switches, and lighting. If your existing electrical panel is full or undersized (100-amp panels are common in older homes), you'll need a panel upgrade ($1,500–$4,000) before adding circuits.

HVAC

Extending your existing HVAC system to cover the new space may require a new zone, additional ductwork, or a supplemental mini-split unit ($3,000–$5,000 installed). If your existing system is undersized for the added square footage, replacing it entirely ($5,000–$15,000) may be necessary.

Permits and Engineering

Building permits for additions typically cost $500–$5,000 depending on your municipality and project value. Structural engineering ($1,500–$5,000) is required for second-story additions and often recommended for ground-level additions. Architectural plans run $2,000–$10,000.

The Permit Process

Home additions require building permits in virtually every jurisdiction. Here's the typical process:

Architectural plans: Hire an architect or designer to create construction documents
Structural engineering: Required for second stories and any structural modifications
Permit application: Submit plans to your local building department
Plan review: 2–8 weeks depending on the municipality
Inspections during construction: Foundation, framing, electrical, plumbing, insulation, final
Certificate of occupancy: Issued after final inspection passes

Skipping permits is illegal and creates serious problems when selling. Unpermitted additions may need to be demolished or retroactively permitted (which often requires bringing the entire addition up to current code).

Planning Your Addition: Key Decisions

Setback Requirements

Every lot has setback requirements—minimum distances from property lines where you can build. Check your local zoning code before assuming you can build in a specific direction. Setbacks typically range from 5 to 25 feet depending on the side of the house.

Lot Coverage Limits

Most zoning codes limit how much of your lot can be covered by structures. If your home already covers 40% of the lot and your code limits coverage to 50%, your addition size is constrained.

Matching the Existing Home

The best additions look like they were always part of the house. This means matching rooflines, siding, window styles, and proportions. Cutting corners on exterior matching saves money during construction but costs you at resale.

Going Up vs. Going Out

Second stories cost more per square foot but don't reduce your yard. Ground-floor additions are simpler but eat into outdoor space. If your lot is small, going up may be your only option. If your foundation can't support a second story without major reinforcement, going out is more practical.

Realistic Timeline for a Home Addition

Design and engineering: 4–8 weeks
Permit approval: 2–8 weeks
Foundation: 1–3 weeks
Framing: 2–4 weeks
Roofing and exterior: 1–3 weeks
Rough-in (electrical, plumbing, HVAC): 2–3 weeks
Insulation and drywall: 1–2 weeks
Interior finish: 2–4 weeks
Final inspections: 1–2 weeks

Total: 4–9 months from design start to move-in for a typical room addition. Second-story additions take 6–12 months.

Return on Investment

Home additions don't typically return 100% of their cost at resale, but they can come close:

Bathroom addition: 50%–60% ROI
Primary suite addition: 50%–65% ROI
Family room addition: 50%–60% ROI
Second-story addition: 55%–70% ROI
In-law suite / ADU: 60%–80% ROI (higher in markets with rental demand)
Sunroom: 40%–50% ROI

The real value often isn't the resale return—it's avoiding the transaction costs of selling and buying. Between agent commissions, closing costs, and moving expenses, changing homes can cost $30,000–$80,000+. If an addition solves your space problem, you keep that money.

How to Hire for a Home Addition

Home additions require a general contractor experienced in new construction, not just remodeling. The contractor needs to coordinate:

Excavation and concrete
Framing
Roofing
Electrical, plumbing, and HVAC subcontractors
Insulation and drywall
Finish carpentry

What to look for:

Licensed general contractor with addition/new construction experience
Portfolio of completed additions (ask to see 3–5 projects)
References you can actually call
Detailed written estimate with line items
Clear payment schedule tied to milestones (never pay more than 10% upfront)
Proof of general liability and workers' comp insurance

Red flags:

Asking for 50%+ upfront
No written contract
Can't provide permit history
Significantly lower bid than competitors (often means they're missing scope)

Frequently Asked Questions

How much does it cost to add a 12x12 room to a house?

A 144-square-foot room addition costs $12,000–$36,000 for a basic bedroom or office. Adding a bathroom increases the cost to $25,000–$55,000. Adding a full en-suite with higher-end finishes can reach $60,000+.

Is it cheaper to add on or build up?

Building out (ground-floor addition) is typically 20%–40% cheaper per square foot than building up (second story) because you avoid structural reinforcement of the existing home. However, building up preserves yard space and doesn't require a new foundation.

How much does a second-story addition cost for a ranch house?

For a 1,000–1,500 sq ft second story on a ranch, expect $150,000–$450,000. The existing foundation almost always needs reinforcement, which adds $10,000–$30,000. Temporary relocation during construction is usually necessary.

Do home additions require permits?

Yes, always. Home additions modify the structure and footprint of your home, which requires building permits in every US jurisdiction. You'll typically need architectural plans, structural engineering, and multiple inspections throughout construction.

Can I live in my house during an addition?

For ground-floor additions, usually yes—though expect noise, dust, and disruption for months. For second-story additions, you'll likely need to move out for 2–4 months during the structural phase when the roof is removed.

How much does an ADU cost to build?

A detached ADU (400–800 sq ft) typically costs $100,000–$250,000. Prefab ADU options range from $50,000–$150,000 including installation. Attached ADUs or garage conversions cost $50,000–$120,000. Costs vary significantly by local labor rates and permitting requirements.

Does adding square footage increase property taxes?

Yes. Your local assessor will reassess your property after a permitted addition, and your property taxes will increase based on the added value. The increase depends on your local tax rate and how much value the addition adds.

Home Addition Cost Guide

Key Takeaways