When mortgage rates in St. Louis are as high as they are now, with the 30-year fixed averaging around 6.5 percent, it becomes significantly more enticing to build an addition instead of buying a new home when you already own one. Sticking with your low mortgage rate might seem like it saves you money, but with the range of costs listed on websites, how much do you actually save by building an addition? Whether you are building an addition in St. Louis for the first or fifth time, here’s what prices are looking like for home renovations in St. Louis in 2026.
What a Home Addition Costs in St. Louis
| Addition type | Cost per square foot | Typical project total |
|---|---|---|
| Bump-out (10×10 or smaller) | $80 to $200 | $12,000 to $20,000 |
| Single-story room addition | $150 to $250 | $60,000 to $100,000 for 400 sq ft |
| Primary suite addition | $200 to $350 | $100,000 to $200,000 |
| Second-story addition | $300 to $500 | $100,000 to $250,000+ |
| Garage conversion | $100 to $175 | $40,000 to $70,000 |
These are market ranges, not quotes. Where your project lands depends on the foundation work required, how easily crews can access your lot, and how much of the existing structure the addition touches.
Materials are the biggest reason those ranges run so wide. An addition is essentially a small house attached to your existing one: concrete for the foundation, framing lumber, roofing that ties into the current roofline, and siding or brick that matches the original exterior, plus windows, insulation, drywall, and flooring. Then come the systems, meaning ductwork to extend your HVAC, wiring (and sometimes an electrical panel upgrade), and plumbing if the plan includes a bathroom. Finish level moves the number most; builder-grade flooring and fixtures sit at the bottom of each range, while custom tile, cabinetry, and high-end windows push the top. Older St. Louis homes add one more wrinkle, since matching century-old brick and trim takes specialty masonry materials and the craftsmen who know how to work with them.
The Costs Homeowners Forget
Many of the costs people forget sit outside the construction itself: design and engineering fees, permits (which vary widely depending on whether you are building in the City of St. Louis or one of the suburbs), historic district review if your home falls within one, and a contingency of 10 to 15 percent for the surprises hiding behind old walls. That doesn’t account for the cost of time, either. Many additions take three to six months to build, and design and permitting, not construction, is usually the biggest variable in that timeline.
Should You Add On or Move?
So, with that said, should you add on or move? The math is more lopsided than most people expect. Selling one home and buying another typically consumes 8 to 10 percent of your sale price in commissions and closing costs, which is $40,000 or more on a $500,000 home before you have gained a single square foot. In high-value, low-inventory areas especially, such as Kirkwood, Clayton, and Ladue, expanding almost always beats the price of trading up to a larger home in the same neighborhood. Factor in swapping a pandemic-era interest rate for one near 6.5 percent, and the addition pulls even further ahead.
The cheapest part of an addition is learning the real numbers before committing. The second cheapest is hiring the right builder to deliver them. Anzac Developments is here every step of the way to ensure your renovation in St. Louis and the surrounding areas goes smoothly. Whether your project fits a general contractor or a design-build firm, our design-build process prices decisions as they are made, and our portfolio of completed projects shows what these budgets actually build. Give us a call or contact us online for a quote today.