Reading the Chicago rezoning record
Every row on the map is one ordinance that changed the zoning map. This page explains the three things each row says about it: which class the land left and entered, which route the case took through City Council, and which direction the change went.
Zoning classes
A Chicago zoning class is a district letter code and a dash number. The letters say what the land is for; the number sets how much can be built, mainly through the maximum floor area ratio (FAR), the total floor area allowed as a multiple of the lot area. A lot of 5,000 square feet in a district with FAR 1.2 allows 6,000 square feet of building. Higher dash numbers also allow more dwelling units per lot and taller buildings.
The table lists every class that appears in the record, with its base FAR from the ordinance's bulk tables and how often it is the class a site left (from) or entered (to). Planned developments and a few other district kinds have no single base FAR; their bulk is set case by case.
Suffix letters such as RT-4A are variants of the base district with the same FAR. "PD" stands for every numbered Planned Development; the record keeps the PD number in the clause text. The last group holds readings the parser took from damaged scans, such as a prefix with its dash number lost; they are kept so the counts reconcile and can be checked against the Clerk's PDF from each record.
Routes
A zoning map amendment is an ordinance. Who brings it, and what it binds, determines how it reads in the record.
- Applicant. Filed by the property owner or their attorney with the Department of Planning and Development, which transmits it to Council. The Clerk lists the sponsor as "Misc. Transmittal". This is the ordinary path for a development that needs a different district than the one it sits in.
- Aldermanic. Introduced by the ward's alderperson rather than an applicant. The Clerk names the sponsor, and the application number carries an A prefix. These are often downzonings or clean-ups the ward initiates, sometimes on land the owner did not ask to change.
- Type 1. An amendment tied to a specific site plan. The approved drawings become part of the zoning, so building something different later needs another amendment. The application number ends in T1. Many Type 1 cases re-adopt the class the site already had, which is why the map shows a same class category: the point was to bind the plan, not to change the district.
Directions
Direction compares the base FAR of the two classes. It is a measure of allowed intensity, not of use: a change from a manufacturing district to a residential one with the same FAR is lateral, and the record notes the use shift separately.
What the map shows and does not
- The pin is the address in the ordinance title, geocoded against Cook County's address points, not the outline of the rezoned land. When an ordinance names several addresses the pin is their average.
- Map No. in a title is the sheet of the city's zoning map grid on which the change is drawn.
- Text source. Most ordinances carry a text layer the parser reads directly. Scanned ones were read by optical character recognition and are marked OCR; the raw strings the parser saw are shown in each record's provenance section so a doubtful reading can be checked against the Clerk's PDF.
- Partial parse. Where only one side of the amendment clause could be read, the row is kept and labelled rather than dropped.