Looking for an addition contractor in O'Fallon, MO? Describe your project once and up to three addition contractors who work in O'Fallon and St. Charles County get back to you with a quote. It's free and there is no obligation.
O'Fallon is home to about 95,355 people (2024 Census Bureau estimate), a growing community (+4.4% since 2020). With 3,121 people per sq mi, it is a suburban area. Growing families here often add space instead of moving.
O'Fallon at a glance
- Population (Census Bureau estimate, 2024)
- 95,355 (+4.4% since 2020)
- Rank in Missouri
- 7th of 938 cities and towns
- Land area
- 31 sq mi
- Population density
- 3,121 people per sq mi (suburban)
- Type
- City (O'Fallon city)
- Main county
- St. Charles County
What to check before hiring in O'Fallon
- Zoning: check O'Fallon's setbacks, lot coverage and height limits first.
- License and permit: in Missouri, additions are built by licensed contractors under a permit.
- Structure: second stories need an engineer to check the existing house.
- Price drivers: size, foundation, roof tie-in, plumbing and finishes.
Useful official resources
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ICC: International Code Council
Official information and guidance.
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NAHB
Official information and guidance.
Services available in O'Fallon
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Room addition
Ground-floor addition with foundation, framing, roof and finishes.
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Second-story addition
New floor over the existing house, with structural engineering.
Frequently asked questions in O'Fallon
How much does an addition cost in O'Fallon, MO?
It depends on size, foundation and finishes. Describe your project: contractors who serve O'Fallon reply with a quote.
How long does it take?
Often a few months, plus design and permits.
Does an addition raise property taxes?
Usually, since it increases the assessed value.
Which addition contractors will contact me?
Up to three independent addition contractors who serve O'Fallon and the surrounding area, chosen for the service you need. Your details are shared only with your consent.
Source: U.S. Census Bureau, Population Estimates Program (Vintage 2024) and Gazetteer Files (public domain), imported on 10/31/2026. Figures describe the incorporated place or county as published; they are shown for information only.