More than 40% of Delaware County homeowners might be paying too much in property taxes this year, and will be next year, too. But that should change by 2021.

As of last week, all homeowners had received letters listing the number of bedrooms, bathrooms, square footage, age, and other details about each home.

The letters look like property assessment records, but they don’t contain a value. Instead, they are meant for owners to check the accuracy of the information that will be used to assign new assessments — which are used to calculate property tax bills — to every property in the county as part of a court-ordered reassessment.

Until the new values take effect in 2021, however, only about 26% of homeowners will be paying taxes based on assessments that are fair, according to an Inquirer analysis of real estate data.

About 33% of homes in Delaware County are underassessed and therefore paying too little in property taxes, and about 41% of assessments are too high, The Inquirer found.

Similar portions of homeowners are paying too much or too little in Bucks, Montgomery, and Chester Counties. But officials in those counties said last week that they are not considering their own reassessments.

» READ MORE: Are you one of more than 200,000 people paying too much in property taxes?

An annual state analysis released last month about each county’s assessments shows that they are continuing to fall out of line with the real estate market, as homes appreciate in value and more time passes since the counties last revalued. Montgomery, Delaware, and Chester Counties have not reassessed their properties in about 20 years; Bucks County last reassessed decades before that.

This week is the deadline to file an appeal in all four counties for homeowners who believe that their current assessments are unfair. Here’s what you need to know:

Why are so many properties over- or underassessed?

State law requires counties to reassess all properties at once. The process is expensive and politically unpopular, so counties are reluctant to undertake it. When decades pass between assessments, that leads to inequities — which led to a judge ruling in 2017 that Delaware County must revalue its properties.

In some municipalities, such as Upper Darby, homes have not appreciated in value relative to other homes in other towns and tend to have assessments that are too high. Others towns, such as Radnor, have had a booming real estate market, so many homeowners are likely paying less than their fair share in taxes compared with the rest of the county.

Assessments in Delaware County currently should be 56.4% of market value, or the amount for which a home would sell, according to the latest analysis, known as a common level ratio, released by the state in June. But thousands of properties are assessed at a higher percentage of value, and thousands are assessed at a lower percentage.

The reassessment in Delaware County will value homes at 100% of their market value, reducing inequities among taxpayers. As time passes after the reassessment takes effect, that ratio will become smaller.

The state calculates common level ratios for every county in the state annually, by comparing assessments to sales prices. The just-released ratios are based on 2018 sales. Philadelphia, unlike other counties in the state, updates assessments regularly so that they are kept as close to 100% of market value as possible; its reassessments are also not revenue neutral, as in other counties, which has contributed to recent criticism and tax increases.

How do you know if you’re paying too much?

To figure out how fairly your home is valued, first look up your home’s current assessment and your county’s common level ratio.

Below are the 2018 ratios that the state released this summer.

  • Montgomery County: 49.3

  • Chester County: 49.3

  • Bucks County: 9.4.

  • Delaware County: 56.4

These numbers are ratios of assessment to sales prices, so if a home sells for $100,000 in Montgomery County, its assessment should be 49.3% of that price, or $49,300. But only about 39% of homes in Montgomery County are assessed close to that ratio, The Inquirer’s analysis found, while 27% are underassessed and 34% are overassessed.

If you recently bought a home, you can compare the sales price to the assessment using the county ratio. If there is no recent sale, you can look at sales prices of similar homes on your street or in your neighborhood.

My house is overassessed. What can I do about it?

The common level ratios also set the appeal standard in each county, meaning that you can successfully appeal if you can show that your assessment is too high compared with the common level ratio.

The deadline to file an appeal is this week; state law sets the appeal deadline each year for Aug. 1.

Property owners can submit appeals to their county board of assessment appeals. Every county has a form for homeowners to submit. They are then assigned hearing dates before the board.

What is next for Delaware County?

Homeowners in Delaware County began receiving letters with information about each property in October, and the mailings continued through last week, according to the county’s website.

The letters allow property owners to notify assessors before values are set if there is inaccurate information. Tyler Technologies, the company that Delaware County contracted with to complete the revaluation, plans to notify taxpayers of their assessments in February and March.

“The reassessment is currently on track and progressing,” Katherine Morrone, a spokesperson for the county, said in an emailed statement.

Property owners will begin paying taxes on the new values in 2021.

The reassessment will be revenue neutral, which means that the county, school districts, and municipalities must adjust the tax rates so they do not collect more money as a result of the new values. But individual taxpayers’ bills could increase or decrease, depending on whether their assessment was previously too high or too low relative to the others in the county.