Windows and siding in St. Charles County
6 places we cover, and the rules that apply in each.
Missouri has no statewide building code and no statewide energy code. Adoption happens municipality by municipality, which is why the table below does not read like one county at all — it reads like 6 separate countries. Between them these places have adopted 3 different energy code arrangements.
25.0% of the housing in St. Charles County was built before 1980, and 2.0% before 1940 (3,321 of 164,594 units). That is the number that decides whether the federal lead rule is on your job, and on both of these trades it very often is: window replacement is never exempt from it at any size, and a whole-house re-side clears the exterior threshold immediately.
| Place | Permit to re-side? | Permit for a like-for-like window? | Adopted energy code | Window must meet |
|---|---|---|---|---|
| Cottleville | Yes | No | 2021 IECC | U-0.30 |
| Lake St. Louis | Yes | Yes | 2015 IECC | U-0.35 |
| O'Fallon | No | No | 2021 IECC | U-0.30 |
| St. Charles | No | Not published | 2015 IECC | U-0.35 |
| St. Peters | Yes | Yes | 2021 IECC | U-0.30 |
| Wentzville | Yes | Yes | 2021 IRC energy provisions, locally amended | U-0.40 |
Things worth knowing before you sign anything
Cottleville
Cottleville's exemption list is the clearest drafting in the St. Charles County group: it explicitly exempts same-opening window and door replacement while conspicuously not exempting siding, so a contractor doing both jobs on one house needs a permit for the siding and none for the windows. Its accessory-structure threshold (120 sq ft) is the lowest in this metro, and it exempts 'window awnings' and 'flatwork' by name. The 2021 IRC adoption carries an unusually long appendix list (A, B, C, E, F, G, H, J, M, N, P). All of it flows from a single 2024 ordinance, No. 2157.
Lake St. Louis
Lake St. Louis also permits asphalt roofing (a dedicated permit guide exists), which is the opposite of the O'Fallon and Jefferson County reroofing exemptions a few miles away. Permit technician for pre-checking whether a job needs a permit: Michelle Debord, 636-625-7937, mdebord@lakesaintlouis.com.
O'Fallon
The codified adoption is explicit - Sec. 500.010 adopts the International Building Code, 2021 Edition, including Appendix Chapter K, and Sec. 500.330 adopts the International Residential Code, 2021 Edition, with Appendices AE, AF, AG, AH, AI, AJ, AK and AP. O'Fallon also runs unusually generous exemptions relative to its neighbours: detached accessory structures up to 200 sq ft, fences up to 7 feet, and asphalt reroofing all permit-free. Residential contact is Scott Dabney, residential supervisor (sdabney@ofallonmo.gov).
St. Charles
The applicable standards are the Secretary of the Interior's Standards: 'Deteriorated historic features shall be repaired rather than replaced. Where the severity of deterioration requires replacement of a distinctive feature, the new feature shall match the old in design, color, texture, and other visual qualities and where possible, materials.' That is the operative constraint on replacing historic wood windows and original siding in the South Main / Frenchtown areas.
St. Peters
St. Peters does not publish any consumer-facing 'do I need a permit' list - the Building Code Guide page only names the adopted chapters and warns that 'Codes are changed regularly,' directing callers to the Planning Department at 636-477-6600 ext. It is also the only St. Charles County city in this metro whose amended R105.2 drops the same-opening window/door exemption that O'Fallon, Cottleville and Jefferson County all carry, which makes it materially stricter on pocket window replacement than its immediate neighbours.
Wentzville
Construction hours are restricted to 6 a.m.-9 p.m.
Windows or siding in St. Charles County?
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