Know Your Budget.
The contractor's quote is about seventy percent of what you will spend.
The rest is design fees, permits, site work, the contingency you will absolutely use, and a tax bill that starts a year after the dust settles. Here it is in the order the money actually leaves.
Compiled from published 2026 Connecticut design-build and cost-guide figures. These are brackets, not quotes. CT sources put additions anywhere from $125 to $550 per square foot depending on room type, site conditions and finish level. Fairfield County runs well above the state average; rural Litchfield and Windham typically run under it.
| Addition | Bump-out to second story | $200–450/sf |
| New build | Builder grade to high-end | $240–425/sf |
| ADU | Conversion to detached new build | $275–450/sf |
| Renovation | Cosmetic to kitchen/bath heavy | $100–300/sf |
A $150,000 addition rarely adds $150,000 to your assessment. Depending on scope, assessors typically recognise somewhere between a third and three quarters of what you spend. The estimator applies a scope-specific multiplier rather than assuming dollar-for-dollar.
Cosmetic work often is not recognised at all. Assessors separate improvements that add value from maintenance that preserves it, and the building permit is usually what puts your property on their list in the first place. Paint, flooring and like-for-like replacements normally clear neither bar. Move plumbing or add a bathroom and it becomes a capital improvement.
| Design & architecture | Full service, through construction admin | 8–12% |
| Permits & fees | Usually a percentage of declared value | 1–2% |
| Site work | More on ledge or slope | 5–15% |
| Contingency | You will use it | 10–20% |
| Property tax increase | Annual, permanent | Forever |
Assessors do not tax a hole in the ground. Most CT towns revalue after the Certificate of Occupancy, and the new assessment takes effect on the following October 1 grand list, so your first higher bill typically arrives twelve to eighteen months after you broke ground.
That lag is a cash-flow gift and a trap in equal measure. Budget for the increase from the day you sign, not the day it shows up. Run the number on the estimator, then see where your town sits on the schedule.