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Building permits in St. Louis County, Missouri

Find the right authority before you build. St. Louis County permitting follows Missouri law — here is exactly who issues the permit and where to start.

County + cities (mixed)

St. Louis County Department of Transportation and Public Works — Permit Processing / Code Enforcement

St. Louis County issues building permits for unincorporated areas through its Department of Transportation and Public Works. Incorporated cities within the county (e.g., Clayton, Chesterfield, Florissant) issue their own building permits, so permit authority depends on whether the property lies within an incorporated municipality.

Open the official permit portal →

Use the county Accela/Citizen Access portal (aca.stlouisco.com) for unincorporated-area permits; contact the relevant city building department if inside city limits.

Permit cheat-sheet (Missouri rules)

Project	Permit?	Authority	Typical fee	Timeline	Go / No-Go	Key notes
🏠 Accessory Dwelling Unit (ADU)	Permit required	city	Permit fees set locally by each municipality — verify with the city building department (St. Louis Building Division or KC Permits Division).	Varies by municipality — simple permits often same-day to ~2 weeks; full plan-review permits several weeks.	Case by case	ADUs require a building permit plus zoning/use approval. Missouri has no statewide residential building code, so allowance and rules are set locally (e.g., St. Louis and Kansas City each have their own requirements). Verify with your municipal building department.
🪵 Deck / Porch	Conditional	city	Permit fees set locally by each municipality — verify with the city building department.	Varies by municipality — over-the-counter/simple permits often same-day to ~2 weeks.	Generally allowed	Kansas City adopted the 2018 IRC (effective 2020); St. Louis enforces the I-Codes locally. Exact thresholds follow local amendments — verify with your city. Exempt: Freestanding decks low to grade and not attached are generally exempt under the locally adopted IRC R105.2; attached decks and decks over 30 in require a permit.
🧱 Fence / Wall	Conditional	city	Usually no building permit under the height threshold; zoning setback/height rules apply. Verify locally.	Varies by municipality.	Generally allowed	Missouri has no statewide code, so municipalities set fence rules individually. Verify with your local building department.
🛖 Shed / Outbuilding	Conditional	city	No fee under the exemption; a permit fee applies if over the local threshold.	Varies by municipality.	Generally allowed	Larger sheds or sheds with utilities require a permit; thresholds vary by city. Verify with your municipal building department. Exempt: Detached one-story accessory shed ≤120 sq ft is exempt under the locally adopted IRC R105.2.
🪜 Basement Finish	Permit required	city	Building permit fee set locally; trade permits billed separately.	Varies by municipality — typically several weeks for full plan review.	Generally allowed	Finishing a basement into habitable space requires permits for egress, framing, electrical, plumbing and mechanical work; a sleeping room needs an egress window.
🚗 Garage / Carport	Permit required	city	Building permit fee set locally; zoning clearance may be required first.	Varies by municipality — typically several weeks for full plan review.	Generally allowed	A new detached or attached garage requires a building permit. Kansas City requires a residential building permit for all new construction and additions to one- and two-family structures.
☀️ Rooftop Solar PV	Permit required	city	Electrical/structural permit fee set locally by the municipality.	Varies by municipality — often expedited for residential PV.	Generally allowed	Rooftop solar PV requires an electrical and (if the structure is modified) building permit, reviewed under the locally adopted code.
🌡️ HVAC Replacement	Permit required	city	Mechanical permit fee set locally by the municipality.	Varies by municipality — often same-day to ~2 weeks.	Generally allowed	Furnace, AC or heat-pump replacement requires a mechanical permit. Kansas City allows licensed contractors to pull permits for HVAC/water-heater/panel replacements without a full plan review.
🍳 Kitchen Remodel	Permit required	city	Building permit fee; trade permits (plumbing/electrical) billed separately.	Varies by municipality — typically several weeks for full plan review.	Generally allowed	A permit is required when relocating plumbing/electrical, moving walls, or changing fixtures. Cosmetic-only work (cabinets, counters, paint) may be exempt.
🛁 Bathroom Remodel	Permit required	city	Building permit fee; trade permits (plumbing/electrical) billed separately.	Varies by municipality — typically several weeks for full plan review.	Generally allowed	A permit is required when relocating plumbing/electrical, moving walls, or changing fixtures. Cosmetic-only updates may be exempt.
🏗️ Room Addition	Permit required	city	Building permit fee based on valuation; zoning and plan review apply.	Varies by municipality — typically several weeks for full plan review.	Generally allowed	Room additions change the footprint and require a full building permit, zoning approval and usually a site plan.
🪟 Window / Door Replacement	Conditional	city	Permit fee if changing the opening; in-kind swap is often exempt.	Varies by municipality — in-kind swaps often same-day.	Generally allowed	Swapping windows or exterior doors in the same opening is generally exempt; enlarging or adding openings requires a permit. Kansas City exempts windows not exceeding 4 ft in load-bearing walls from plan review.
🏘️ Roof Replacement	Conditional	city	Roof-covering permit fee set locally; in-kind re-roof may be exempt.	Varies by municipality — in-kind re-roofs often same-day.	Generally allowed	Roof covering replacement is typically permitted; structural deck replacement triggers a full building permit.
🅿️ Driveway / Parking Pad	Conditional	city	No building permit for private flatwork; a right-of-way/curb cut needs a separate permit.	Varies by municipality.	Generally allowed	Kansas City requires a permit only for a drive approach located in the public right-of-way. Other municipalities may differ — verify locally.
🏊 Swimming Pool / Spa	Permit required	city	Building permit fee; barrier fencing required by code.	Varies by municipality — typically several weeks.	Generally allowed	Pools need a permit and barrier fencing. Kansas City issues residential swimming-pool permits; verify your municipality’s threshold and barrier rules.
🧱 Retaining Wall	Conditional	city	No fee under the exemption; a permit applies if over the threshold.	Varies by municipality — if required, typically several weeks.	Generally allowed	Walls over 4 ft or supporting a surcharge require design and a permit. Verify with your municipal building department. Exempt: Retaining walls ≤4 ft (not supporting a surcharge) are exempt under the locally adopted IRC R105.2.

Sources: St. Louis County — Permits & Code Enforcement (Dept. of Transportation and Public Works) · American Wood Council — Missouri Adopted I-Codes (local adoption; no statewide mandate) · City of St. Louis — Building Division · City of Kansas City, MO — Development Process Guide (Permits Division)