O'Fallon, MO Building Safety: Building Permits (2026)

390 makes it unlawful to alter real property inside the city limits, including driveways, fences, additions to homes and businesses, and any other building activity regulated by city ordinance or code, without first obtaining the proper permit. The Building Official or a designated representative enforces this requirement citywide under Subsection B. 2 to carve out a defined exemption list so residents are not forced to permit trivial work. Exempt building work includes retaining walls up to 4 feet tall (unless they support a surcharge or hold back Class I, II or IIIA liquids), water tanks on grade holding no more than 5,000 gallons, sidewalks and driveways no more than 30 inches above grade, painting and similar finish work, temporary movie and stage sets, prefabricated above-ground pools under 24 inches deep and 5,000 gallons, shade cloth structures for farming, small window awnings, movable fixtures and racks under 5 feet 9 inches tall, flagpoles under 35 feet, like-for-like door and window replacement, and asphalt shingle reroofing.

Electrical, gas, mechanical and plumbing work has its own parallel exemption list covering minor repairs, portable appliances and fixture swaps that do not relocate connections. Everything else, from new construction to additions, needs a permit reviewed and issued through the Building and Code Enforcement Department before work starts.