St. Charles County

Home permits and project costs in O'Fallon, MO

O'Fallon sits in St. Charles County, Missouri. Freeze-thaw cycling, frost-depth footings and an older housing stock keep foundation, sewer and window work in steady rotation. Work outside city limits is permitted by the county building department rather than the city.

Verified August 2026

Local cost index

-10%

vs national average

Permit fee index

-15%

vs national baseline

Population

91,316

Homeownership

Do you need a permit in O'Fallon?

For most structural work, yes

O'Fallon enforces local adoption of the irc; no statewide residential code through St. Charles County. Roofing, electrical, plumbing, structural, pools and most exterior additions require a permit and inspection. Cosmetic work — paint, flooring, cabinet swaps — generally does not.

Project costs in O'Fallon

National baselines adjusted by the local labor and materials index.

Project	Typical cost	Permit	Deep pages
Roof Replacement	$8,700 – $29,200	required	Cost·Permit
AC Replacement	$4,800 – $14,200	required	Cost·Permit
Fence Installation	$2,200 – $10,000	usually required	Cost·Permit
Water Heater Replacement	$1,100 – $4,900	required	Cost·Permit
Pool Installation	$34,500 – $104,300	required	Cost·Permit
Window Replacement	$6,900 – $31,300	required	Cost·Permit
Electrical Panel Replacement	$1,700 – $6,200	required	Cost·Permit
Generator Installation	$5,900 – $19,900	required	Cost·Permit
Solar Panel Installation	$14,000 – $44,700	required	Cost·Permit
Kitchen Remodel	$16,500 – $84,500	conditional	Cost·Permit
Bathroom Remodel	$8,200 – $38,400	conditional	Cost·Permit
Driveway Replacement	$3,800 – $17,200	usually required	Cost·Permit
Paver Patio & Walkway	$3,200 – $14,500	conditional	Cost·Permit
Deck Construction	$5,000 – $25,400	required	Cost·Permit
Shed Installation	$1,600 – $10,900	conditional	Cost·Permit
Whole-House Repipe	$4,000 – $14,300	required	Cost·Permit
EV Charger Installation	$700 – $3,800	required	Cost·Permit
Hurricane Shutter Installation	$2,900 – $16,400	required	Cost·Permit
Pool Enclosure / Screen Cage	$8,200 – $38,100	required	Cost·Permit
Septic System Replacement	$6,800 – $25,500	required	Cost·Permit
Impact Window Installation	$8,300 – $38,800	required	Cost·Permit
Exterior Door Replacement	$800 – $5,900	usually required	Cost·Permit
Garage Door Replacement	$1,300 – $6,800	required	Cost·Permit
Siding Installation	$7,300 – $34,500	usually required	Cost·Permit
Stucco Repair	$800 – $14,600	conditional	Cost·Permit
Soffit & Fascia Replacement	$1,600 – $10,000	conditional	Cost·Permit
Gutter Installation	$800 – $5,900	rarely required	Cost·Permit
Roof Repair	$400 – $4,300	conditional	Cost·Permit
Flat Roof Replacement	$5,500 – $25,500	required	Cost·Permit
Attic Insulation	$1,100 – $7,000	rarely required	Cost·Permit
Ductwork Replacement	$2,300 – $11,000	required	Cost·Permit
Mini Split Installation	$2,200 – $11,500	required	Cost·Permit
Flooring Installation	$2,300 – $18,300	rarely required	Cost·Permit
Tile Installation	$1,600 – $14,500	rarely required	Cost·Permit
Interior Painting	$1,300 – $8,500	rarely required	Cost·Permit
Exterior Painting	$2,500 – $13,400	rarely required	Cost·Permit
Drywall Repair	$300 – $4,000	rarely required	Cost·Permit
Kitchen Cabinet Refacing	$4,100 – $18,100	rarely required	Cost·Permit
Countertop Replacement	$2,000 – $11,100	rarely required	Cost·Permit
Tub to Shower Conversion	$3,200 – $14,400	usually required	Cost·Permit
Sod Installation	$800 – $6,400	rarely required	Cost·Permit
Irrigation System Installation	$2,000 – $9,100	conditional	Cost·Permit
Tree Removal	$400 – $4,900	conditional	Cost·Permit
Outdoor Kitchen	$5,400 – $40,800	usually required	Cost·Permit
Seawall Repair	$7,200 – $85,900	required	Cost·Permit
Dock Construction	$8,100 – $76,800	required	Cost·Permit
Well Installation	$5,000 – $22,600	required	Cost·Permit
Mold Remediation	$1,300 – $13,400	conditional	Cost·Permit
Water Softener Installation	$1,100 – $5,900	usually required	Cost·Permit
Home Addition	$40,900 – $254,500	required	Cost·Permit

O'Fallon permitting basics

Jurisdiction	O'Fallon, St. Charles County
Building department	Confirm with the city or county
Permit portal	No verified online portal on file
Owner-builder permits	Homeowner permits widely allowed
Contractor verification	Verify any license with Missouri Division of Fire Safety before signing

Sources

Sources & verification

Missouri Division of Fire Safety

Last verified August 2026. Cost figures are modeled estimates based on national baselines adjusted for local labor and material pricing — they are not quotes. Permit rules change; confirm with the building department before starting work.