Why Do New Construction Property Taxes Increase After You Move In?
New construction property taxes in Illinois usually start low because the county assesses real estate as of January 1 each year, often before your home is finished or occupied. Once the assessor picks up the completed home, typically one to two years later, your bill rises to reflect its full value. Illinois also offers a four-year Homestead Improvement Exemption that can soften part of that increase, and most new-construction buyers never find out it exists until it’s too late to plan around it.
By Kealan O’Neil | September 21, 2026
If you’re closing on a new-construction home in Yorkville, Oswego, or Montgomery this year, the tax figure on your closing statement is not the number you’ll be paying once your assessment catches up. New construction property taxes in Illinois work on a delay, and that delay catches more buyers off guard than almost anything else in the process.
It’s one of the most common points of confusion I run into, and it shows up constantly outside the Fox Valley too. Go looking for it and you’ll find buyers across the country asking the same nervous question: is my tax bill really going to jump once the county “finds” my finished house? In Illinois, the honest answer is usually yes, and the size of the jump depends on how far behind the assessment was when you closed.
Why Your First Tax Bill Isn’t the Real One
Illinois assesses real estate as of January 1 every year. That date, not your closing date and not your certificate of occupancy, is what the Illinois Department of Revenue calls the start of the assessment cycle for all real property in the state.
If your home wasn’t finished, or wasn’t finished by much, on the January 1 that applies to your first bill, the assessor has nothing to value but land and whatever partial construction existed. Some homes get valued on a completion-percentage basis: a house that’s 40% built toward a $600,000 finished value gets assessed closer to that partial number, plus the land. Either way, the number on your builder’s estimate and the number on your first county bill are often not the same thing, and neither one is your long-term number.
This is a different issue from the property tax proration credit you’ll see on your closing statement, which is about how the seller (in this case, usually the builder) credits you for taxes not yet billed on the prior owner’s assessment. I’ve written about how that proration math works separately — worth reading alongside this one, because buyers often conflate the two and they’re solving different problems.
The Catch-Up: What Actually Happens in Year Two or Three
Once the county’s assessor cycles back through and picks up the finished home, whether that’s the following year or the year after, your assessed value jumps to match reality. Illinois assesses most property at 33.33% of market value, per Kendall County’s assessments office, so the gap between “land and a foundation” and “a finished $500,000 house” translates directly into a gap between your first bill and your real one.
This isn’t a hypothetical. A Chicago-area real estate blog documented a Cook County case where a buyer’s bill went from roughly $16,000 a year to nearly $30,000 once the assessment caught up, a jump of about $14,000 in a single cycle. Cook County isn’t Kendall or Kane, and your number won’t match that one, but the mechanism is identical statewide because it comes from Illinois’ assessment calendar, not a local ordinance. It’s also why national coverage of this exact pattern, buyers blindsided by a much larger bill once their new build gets fully assessed, keeps showing up market after market, not just in Illinois.
That’s the scenario I want you to plan for before you sign, not after your first real bill shows up. If you’re financing at the edge of your comfort zone based on a builder’s estimated tax figure, you’re financing against a number that’s likely to move.
The Four-Year Exemption Most New-Construction Buyers Never Ask About
Here’s the part almost nobody mentions: Illinois has a Homestead Improvement Exemption built specifically for situations like this. Under state law, up to $75,000 in fair cash value of new improvements to your home, about $25,000 in assessed value, is exempt from the increased assessment for four years after the work is completed, according to the Illinois Department of Revenue’s property tax relief guidance.
That exemption doesn’t erase the catch-up entirely, especially on a home valued well above that threshold, but it takes real weight off the jump in years one through four, and it’s not automatic in every case. Don’t assume your builder, your title company, or your assessor applied it for you. Ask, and if you’re in Kane County, check the county’s homestead exemption page to see what you need to file and by when.
Building This Into Your New-Construction vs. Resale Decision
This is exactly the kind of number that belongs in your decision, not just your closing paperwork. A resale home in Yorkville or Oswego usually comes with a stable, known assessment. A new-construction home comes with a tax bill that’s almost guaranteed to increase, sometimes significantly, within a year or two of you moving in.
If you’re looking at new construction in an area with a Special Service Area, like several of the subdivisions in Yorkville, that’s a separate cost layered on top of this one, not a substitute for it. I’ve broken down how SSA and backup SSA taxes work in Yorkville and Montgomery separately, and it’s worth reading both posts together before you commit to a new-construction lot. If you’re weighing the broader trade-offs of building versus buying an existing home, I’ve also written about new construction versus resale in the Fox Valley.
This is exactly the kind of number I run for clients before they write an offer on a new-construction home, because the payment you qualify for in year one and the payment you’ll actually owe in year three can be two different mortgages. Your specific number depends on the subdivision, the township, the builder’s timeline, and whether the exemption gets applied, and the only way to know for sure is to run it with someone who’s done this for other Fox Valley buyers before you.
Common Questions About New Construction Property Taxes
Buyers building in Yorkville, Oswego, and Montgomery ask me these questions constantly. Here are the straight answers.
Will my new construction property taxes really double?
How long until my assessment catches up to my home’s full value?
Does the Homestead Improvement Exemption apply automatically?
Is this the same thing as SSA taxes on new construction?
Who do I contact to check my new home’s assessment status in Kendall or Kane County?
If you’re weighing a new-construction lot in Yorkville, Oswego, Montgomery, or anywhere else in the Fox Valley, get the real tax picture before you go under contract, not after your first full bill arrives. I’m happy to run the numbers with you. Call or text Kealan at 630-425-8815.
About Kealan O’Neil
Kealan O’Neil is the Designated Managing Broker and founder of O’Neil Property Group, an independent real estate brokerage based in Yorkville, Illinois. He and his team help buyers and sellers in Yorkville, Oswego, Montgomery, and the surrounding Kendall and Kane County communities, and he owns and manages a local rental portfolio, so he approaches every decision as both a broker and an investor. Call or text Kealan at 630-425-8815.
This post is general information about real estate in the Fox Valley, not legal, tax, or financial advice. Real estate laws, tax rules, and municipal ordinances change, and how they apply depends on your specific situation. Talk with a licensed attorney, CPA, or lender before making decisions about a purchase, sale, or tax matter. O’Neil Property Group is a licensed Illinois real estate brokerage.