Do I Need a Permit for a Home Addition?

A home addition always needs a building permit. Whether it is a 6-foot kitchen bump-out, a family room off the back, a second story, or a glassed-in sunroom, an addition makes the house bigger, puts new loads on the ground and on the existing structure, and changes how the house sits on the lot. That is the kind of work building departments exist to review.

Additions also involve more than one approval. Zoning has to confirm the new footprint and height fit the lot, the building code governs structure and energy, and trade permits cover the wiring, plumbing, and ductwork. Use this guide to understand the typical requirements, then confirm with your local office. If you don’t know which office covers your address, you can find your local building department.

What the model code says

The International Residential Code (IRC), which most states adopt for one- and two-family homes, is direct about additions. Section R105.1 requires a permit from anyone who intends to “construct, enlarge, alter, repair, move, demolish or change the occupancy of a building or structure.” An addition enlarges the building, so it is covered.

The exemption list in R105.2 does not help. Its only size-based building exemption (item 1) is for one-story detached accessory structures with a floor area of no more than 200 square feet. Some jurisdictions use a smaller limit. Either way, it covers freestanding sheds and similar buildings, not rooms attached to the house.

Other IRC sections shape what an addition must meet:

• R102.7.1: Additions must meet the requirements for a new structure, without requiring the existing house to comply. An addition cannot make the existing house less compliant than it was, and the house plus addition together must stay within the code’s height limits.
• R106.2: Permit applications must include a site plan showing the new construction, existing structures, and distances from lot lines.
• R403.1.4: Exterior footings must be at least 12 inches below undisturbed ground and, under R403.1.4.1, protected from frost, usually by extending below the frost line your jurisdiction sets in Table R301.2.
• R314.2.2 and R315.2.2: When an addition requiring a permit is built, the whole dwelling unit must get smoke alarms, and carbon monoxide alarms where required, in the locations required for new homes. The exceptions cover exterior-only work like roofing, siding, windows, or adding a porch or deck. They don’t cover a room addition.
• R313.2: New one- and two-family homes need fire sprinklers under the model code. There is an exception for additions to existing homes that don’t already have sprinklers. If your house is already sprinklered, plan on extending the system. Many states remove or change R313, so check your local rule.
• Chapter 11, N1110 (IECC R502): Additions must meet the energy code for new construction. The existing house doesn’t have to be upgraded, but an addition can’t overload existing systems. The code accepts compliance by the addition alone, by the house and addition evaluated together, or by showing that the expanded house uses no more energy than before.

Projects that count as additions

Expect to need a building permit, plus zoning review, for any of these:

• Room additions and bump-outs of any size, including cantilevered bump-outs with no new foundation.
• Second-story or partial second-story additions, and dormers that add floor area.
• Sunrooms and enclosing an existing porch or deck with walls and windows. The IRC defines a sunroom as a one-story structure attached to a dwelling with glazing over 40 percent of the gross area of its exterior walls and roof.
• Attached garages, and converting an attached garage or porch into living space. Those are technically alterations or changes of use, but they get similar review.
• Covered porches and roofed decks attached to the house.

What reviewers and inspectors check

Structure. For a ground-level addition, reviewers look at footings and foundation, floor framing, wall bracing, headers, roof framing, and the connection to the existing house. The IRC requires a complete load path from roof to foundation (R301.1). Anything outside the prescriptive limits needs engineered design (R301.1.3). For a second story, the key question is whether the existing footings, foundation walls, and first-floor framing can carry the new loads. Many departments ask for an engineer’s evaluation, and some cities route these projects to full plan review automatically.

Energy. New walls, ceilings, floors, windows, and ducts in the addition must meet your locally adopted energy code. Some departments ask for a compliance form or checklist with the application. A new HVAC system or extended ductwork may also bring duct leakage testing.

Life safety. Bedrooms in the addition need emergency escape openings. Smoke and CO alarms must be brought up to new-home locations throughout the dwelling unit. Find out if they must be hardwired and interconnected where the walls are open.

Inspections. IRC R109.1 calls for foundation, frame and rough-in, and final inspections. In practice, an addition usually sees footing or foundation, under-slab or under-floor, plumbing, electrical, and mechanical rough-in, framing, insulation, and final inspections. See building inspection stages for how scheduling usually works.

Zoning limits come first

A design that passes the building code can still fail zoning. Before paying for full drawings, check the following:

• Setbacks from front, side, and rear lot lines. Additions are often blocked by side setbacks on narrow lots.
• Lot coverage, or the share of the lot covered by buildings.
• Floor area ratio (FAR) or gross floor area caps, where they exist.
• Height limits, which matter for second stories and dormers.
• Easements, floodplains, and protected areas such as stream buffers.
• Historic district and HOA approval for visible exterior changes.

Most departments need a survey or plat of the lot so reviewers can confirm the distances.

How the rules differ by city

Montgomery County, Maryland. The Department of Permitting Services’ guidance sheet for the common R-60 zone lists a 35 percent maximum lot coverage including accessory buildings, setbacks of 25 feet front, 8 feet on one side and 18 feet total for both sides, 20 feet rear, and a height limit of 35 feet to the roof peak. Under the same sheet, an addition of more than 50 percent of the existing floor area on an older lot (platted before 1978 and under 25,000 square feet) counts as infill development. Infill development uses a stricter coverage limit: 30 percent on lots under 6,000 square feet, sliding down to 20 percent on lots of 16,000 square feet or more.

Fairfax County, Virginia. Residential addition permits are filed online in PLUS. The application includes a county coversheet, architectural and structural drawings, and either a house location plat or a grading plan record number. Trade permits are issued after the building permit. Which site plan you need depends on how much land you disturb. A house location plat covers projects disturbing no more than 2,500 square feet. A conservation plan can be used for small additions disturbing 2,500 to 5,000 square feet. Large additions disturbing more than 2,500 square feet need an infill lot grading plan. On fees, the county schedule effective July 1, 2026 charges $0.15 per square foot for wood-frame (Type V) residential additions. The area counts all floors, including basements and horizontally projected roof areas. The minimum permit fee is $141.75, and a 2 percent Code Academy levy and a 10 percent technology surcharge are added on top.

Seattle, Washington. SDCI Tip 316 lets some additions use a faster subject-to-field-inspection (STFI) permit. To qualify, an addition must be ground-level and one story, no more than 750 square feet, with structural beam spans of 14 feet or less (30 feet with manufactured trusses), and it must meet the prescriptive energy code. Dormers qualify only if they are 14 feet wide or less, add no more than 200 square feet, and have stamped structural plans. Second-story and two-story additions are excluded from STFI and go through full plan review.

Denver, Colorado. Additions need three permits: zoning, building, and a sewer use and drainage permit (SUDP). Uploading everything for all three at once gets simultaneous review. Since May 22, 2025, trade permits for additions require an issued residential construction permit first. Additions over 500 square feet of gross floor area are subject to the city’s construction and demolition recycling rules, and the building permit won’t issue until a waste diversion plan is approved and a refundable deposit is paid.

San Diego, California. The city’s Information Bulletin 146 exempts residential construction that adds no more than 500 square feet of assessable space from school fees. Larger additions pay. Garages, decks, patios, and solariums are not assessable space. Within San Diego Unified, the residential rate is $5.38 per square foot effective May 10, 2026. The district issues a certificate of compliance once the fee is paid.

What to have ready when you apply

• Survey or plat showing lot lines, existing structures, and easements. Ask whether an old closing plat is acceptable.
• Site plan with the addition’s footprint, setbacks, lot coverage, impervious surfaces, and drainage.
• Floor plans for the addition and the affected parts of the existing house, with room uses and window sizes.
• Elevations and a building section showing height, roof pitch, and how the roof ties into the existing roof.
• Foundation and framing plans, including footing size and depth, beam and header sizes, and connections to the existing house.
• Engineer’s letter or stamped drawings for second stories, long spans, or anything outside prescriptive limits.
• Energy compliance documentation in the format your department uses.
• Trade permit information for electrical, plumbing, and mechanical work, and any load calculation for an electrical service upgrade.
• HOA or historic approval if it applies.
• Contractor license information, or an owner-builder affidavit if you are pulling the permit yourself. See owner-builder permits.

Fees and review times for additions are usually much higher than for a remodel. Our guides to building permit cost and how long a building permit takes explain how departments calculate both.

Questions to ask your building department

• What zoning district am I in, and what are the setback, coverage, FAR, and height limits?
• Do I need a new boundary survey, or will an existing plat work?
• Does a second story or large addition need an engineer’s evaluation of my existing foundation?
• What frost depth must the new footings reach?
• Which energy code compliance path and forms do you accept for additions?
• Are smoke and CO alarms required to be hardwired and interconnected throughout the house?
• Does my addition trigger impact fees, school fees, stormwater review, or a grading plan, and at what size?
• Can trade permits be filed with the building permit, or only after it issues?

If an addition was built without a permit, the department will generally require a retroactive permit and may ask you to open walls for inspection. Our guide to unpermitted work covers how that typically works.