If there's one thing I've learned from reading 10s of 1,000s of comments on real estate websites, it's this: few people know where to find public data about homes that are listed for sale, and very few of those few know how to interpret the data they find.
How to interpret the data intelligently is a subject too complex for this post. All this post can hope to do is tell you what public data is available where, highlight some of the limitations of the data, and caution you that Alexander Pope had it right.
A property's 14-digit PIN number
Every home and condo in Cook County has at least one unique 14-digit Property Identification Number (PIN). The Cook County Clerk's site has a useful description of the PIN system, and you should read it. You'll need to know a property's PIN to find information at most public sites.
It's important to be aware that a property may have multiple PIN numbers – e.g. one for a condo and one for a deeded parking spot, or one for a single-family home and one for an adjacent lot that's part of the property.
For-sale listings at Zillow often include a property's PIN number, but it's frequently entered incorrectly – sometimes deliberately. Don't rely on the PIN you find at Zillow.
The best way to find a property's PIN is through an address search at the Cook County Assessor's site. The Assessor's office also has property photos, which will help verify that you've found the correct PIN.
Cook County Assessor's site
The Cook County Assessor determines the assessed valuation, for real estate tax purposes, of every parcel in Cook County. Consult the FAQ's at the Assessor's site for answers to a lot of your tax questions. As a general proposition, properties in Cook County are re-assessed every three years.
A property's address as it's listed for sale might not correspond with the address you'll find by a PIN search at Assessor's site. If you don't find the exact address, try an address range search.
Once you've found a property's PIN you can find the property's assessed valuation for tax purposes, and its estimated market value. Ignore the estimated market value – it often bears no relationship to the property's actual market value. The summary description of the property is often inaccurate in a number of respects, for a variety of reasons beyond the scope of this post.
You can also see the property's exemption history and its appeals history. A pending appeal may or may not result in a reduction in the assessed valuation.
Property taxes in Cook County are paid a year behind, i.e. the taxes payable in 2012 are based on the assessed valuation for 2011.
Cook County Recorder's site
The Cook County Recorder of Deeds maintains an online database of every document recorded against a property, e.g. deeds, mortgages, contractor liens, tax liens, foreclosure filings, etc.
A property's entire history is not online. To see the entire history you'd have to visit the Recorder's office. Summary data covering a limited time period is free online, but you'll have to pay for full copies of available documents.
Explaining the meaning of recorded documents is well beyond the scope of this post. The most common information you'll be looking for are deeds, mortgages and lis pendens (foreclosure) filings.
Deeds will generally tell you how much a property sold for, and when. Mortgage recordings will tell you the amount of the original financing on the property and any refinancing. A lis pendens notice generally, though not always, corresponds to a mortgage foreclosure. The lis pendens notice frequently contains a reference to a case number which you can research at the Cook County Clerk's site (see below).
Transfers of ownership are typically represented by Warranty Deeds, sometimes by Trustee Deeds. Click on a deed to see the sale amount. You should generally ignore Quitclaim Deeds. The Grantor in a deed is the legal seller. You can research whether any litigation is pending against the seller in Cook County at the Cook County Clerk's site.
The mortgages recorded against a property indicate the amount financed at the time of a purchase or refinancing. A Release is usually recorded when the mortgage is paid off, but often isn't. Keep in mind that mortgage terms vary and that people often make payments in addition to those required, so the recorded instruments are not a reliable guide to the current mortgage balance.
If you have a case number from the Recorder's office, you can search foreclosure filings by case number. You can also search by a defendant's name – the current owner as reflected in the records at the Recorder's site. Foreclosure cases are filed in the Chancery Division. Be aware that many names are misspelled at the court filing or data entry level, and that there are many people with identical or similar names.
A search at the Clerk's site only returns an item-by-item search of filings in a case. You can generally glean the status of the case if you're roughly familiar with court documents. You can physically visit the Clerk's office if you wish to read an entire file.
Property tax amount, payment status
You can find the most recent property tax bills at the Cook County Treasurer's site. Enter the property's 14-digit PIN, type in the security code and press Search. The resulting screen will show you the amount of taxes currently due and payable. Click the option to see detailed tax information.
Verify that the Property Location shown matches the address you're researching. The Mailing Location is where the tax bill is sent – it may or may not be the owner's name and address. You'll also see whether Homeowner and Senior Citizen Exemptions were claimed for the property.
Cook County property taxes are payable in two installments, in the spring and fall of the year. The resulting screen shows you the amount of each installment, when it was due and when it was paid. The total of the installments is the total tax bill for the property.
You may see only the first installment. The amount of the first installment is an estimate for the year, based on 55% of the prior year's tax bill. Depending on a variety of factors, the second installment may be more (sometimes substantially more) or less than the first.
New construction or renovation of any significance requires a building permit. You can check the status of building permits at the City of Chicago's website. There's a help page to assist you in interpreting the data.
If a property has multiple addresses it's common practice for developers to use one address for marketing purposes and another for building permit purposes. You can speculate on your own why this might be the case.
Permit data for only the last 18 months is online, and it's said to be updated nightly. If you don't find a permit and work is underway or has been recently completed, that's a big warning flag. The permit is also required to be displayed at the property while work is underway.
Building code violations
A hat tip to Vik Wadhwa of Urban Living for reminding me of the City of Chicago's Building Violations Online site. In addition to information about code violations, this site also returns details of older building permits. If the search comes up empty for code violations, that's no guarantee that there aren't any, only an indication that the city hasn't cited the property for violations.
A property's zoning classification may be the single most frequently misstated piece of information in real estate listings. Few real estate agents make the effort to develop the expertise necessary to understand this highly-technical area. Consult an experienced zoning attorney if you have questions.
Neighborhood school boundaries
One of the most critical pieces of information about a property is often its neighborhood schools. There can be large variances in a property's value from one side of the street to the other based on neighborhood school attendance boundaries.
Under no circumstances should you rely on the neighborhood school information contained in a real estate listing. Always verify the information from the district school board site.
In the City of Chicago, use the Chicago Public Schools School Locator service.
The City's Chicago Landmarks site has an alphabetical listing of landmark districts and landmark sites. That listing is not always up-to-date and it's not easy to identify addresses from the list.
The Landmarks site also links to a database of more than 17,000 historically significant structures.
A property may be listed on the National Register of Historic Places and not be listed as a Chicago Landmark. It is, unfortunately, extremely difficult to search the National Register database to determine whether a property is listed on it.
Landmark status is another category of information that's ill-understood and often misstated by real estate agents. Agents are often confused by the distinction between properties that have independent landmark status and properties that are part of a landmark district and deemed significant to the district.
EveryBlock is an extremely useful source that aggregates a wide variety of local information, including crime data, news stories, blog posts, zoning requests, building permits and more at an address, block, neighborhood or ZIP code level.
A note on Zillow
You'll find PINs, tax amounts and prior sales information on many listings at Zillow, but the public data sources discussed here are more accurate. Zillow is most useful for prior listing history, including price reductions and market times.
A Google search on a property's address often surfaces further information about prior listing and pricing history.