ChicagoGuide

The standard Chicago lot

25 × 125 feet, 3,125 square feet. Here is what it yields in every residential district — units, floor area, height and the ground you may actually build on.

Every figure is computed from the adopted ordinance by the same engine behind the Chicago zoning calculator, so this page and that tool cannot disagree. Rules link to Title 17 in our hosted zoning code.

The lot

Why 3,125 square feet

Most of Chicago’s residential blocks were platted in 25-foot widths on 125-foot-deep blocks, which is why so much of the city sits on the same rectangle. It is common enough that the zoning ordinance reaches for it when it needs an example:

A 25 by 125 foot Chicago lot in RS-3: a 20 foot front yard, a 35 foot rear yard and 5 feet of side yards combined, leaving a buildable area of 20 by 70 feet. 20 ft35 ft25 ft20 × 70street
The same lot in RS-3, the city’s most common district: a 20 ft front yard §17-2-0305, a 35 ft rear yard — the lesser of 50 ft or 28% of depth §17-2-0306 — and side yards totaling 20% of the lot’s width §17-2-0309. What is left is 20 × 70 ft: 1,400 sq ft of ground per floor, on a 3,125 sq ft lot.

The front setback shown is the 20 ft a vacant lot is assumed to owe. In practice it is the average of the nearest lots on either side, so a block of shallow front yards lets you come forward §17-2-0305.

District by district

What 3,125 sq ft allows

The same piece of ground, walked up the residential ladder. Units come from the district’s minimum lot area per unit; floor area is the lot area times the district’s floor area ratio; the buildable area is what the required yards leave behind.

DistrictUnitsFAR Floor areaHeight Buildable
RS-1 — 0.5 1,563 30 ft 15 × 70
RS-2 — 0.65 2,031 30 ft 17 × 70
RS-3 1 0.9 2,813 30 ft 20 × 70
RT-3.5 2 1.05 3,281 35 ft 20 × 72.5
RT-4 3 1.2 3,750 38 ft 20 × 72.5
RM-4.5 4 1.7 5,313 45 ft 20 × 72.5
RM-5 7 2 6,250 45 ft 20 × 72.5
RM-5.5 7 2.5 7,813 47 ft 20 × 72.5
RM-6 10 4.4 13,750 No limit* 25 × 72.5
RM-6.5 10 6.6 20,625 No limit* 25 × 72.5

On a 25 × 125 ft lot. Floor area in square feet, buildable area in feet — the ground inside the required yards, before any encroachment §17-17-0309. *Above a threshold size these districts require a planned development, which sets its own limits. RS-1 and RS-2 show no unit count because this lot is below their minimum lot area — see below.

RS-3

The same 25 by 125 foot lot zoned RS-3: 1 dwelling unit and up to 2,813 square feet of floor area, in a building up to 30 ft. 1 unit

1 unit · 2,813 sq ft · 30 ft

RT-4

The same 25 by 125 foot lot zoned RT-4: 3 dwelling units and up to 3,750 square feet of floor area, in a building up to 38 ft. 3 units

3 units · 3,750 sq ft · 38 ft

RM-5

The same 25 by 125 foot lot zoned RM-5: 7 dwelling units and up to 6,250 square feet of floor area, in a building up to 45 ft. 7 units

7 units · 6,250 sq ft · 45 ft

One lot, three districts, drawn to the same scale. Heights are exaggerated ×1.15 against the lot’s depth so the difference reads; the massing is illustrative, not a design.

What binds

Which limit runs out first

Three numbers cap a building and they do not run out together. Whichever is reached first is the one that decides the project, and on a lot this narrow it is rarely the one people quote.

  • Units — lot area divided by the minimum lot area per unit, rounded down §17-17-0304. In RT-4 that is 3,125 ÷ 1,000 = 3 units.
  • Floor area — lot area times the district’s FAR §17-17-0305.
  • The ground itself — the required yards, which on a 25 ft lot take both sides and 55 ft of the depth.

That last one is the constraint the tables never show. In RS-3 the floor area cap is 2,813 sq ft and the pad is 1,400 sq ft, so you need roughly two full floors to use the allowance — comfortable under a 30 ft limit. Go up the ladder and it inverts: in RM-6.5 the cap is 20,625 sq ft on a pad of 1,813 sq ft, which is 11 floors of coverage. Long before that the parking and open-space rules, and the planned-development threshold, take over as the real limits.

Undersized for its own district

RS-1 and RS-2 require more lot area than a standard lot has — 6,250 sq ft in RS-1 and 5,000 sq ft in RS-2. That does not make the lot unbuildable. A lot lawfully platted before the standard applied is a nonconforming lot, and in a residential district it may still be developed with a detached house §17-15-0202-A — while meeting every other bulk standard §17-15-0203. It does mean the second unit is off the table without relief; see variations and rezoning and nonconforming lots and buildings.

Then the overlays

What the district is not telling you

Everything above is the base district on an ordinary lot. Four things routinely move it, and each one is decided by where the lot is rather than by what it is zoned:

  • Near transit, the parking minimum can fall to zero and floor area and density premiums come into reach — transit-served development.
  • Inside the 606 or Pilsen boundaries, the block face decides whether a detached house may be built at all — the block rules.
  • An additional dwelling unit — a coach house or a conversion unit — can add a unit the density table does not count §17-9-0131; see ADUs and coach houses.
  • Above a threshold, or where the district itself is the obstacle, the project becomes a planned development or a rezoning, and the numbers here stop applying.

For a specific address rather than a standard one, the zoning calculator takes any lot size and district, and the neighborhood guides show which districts actually cover the ground where you are looking.

Questions

Common questions

What is a standard Chicago lot?

25 feet wide by 125 feet deep — 3,125 square feet. It is the platting standard behind most of the city’s residential blocks, and it is the size the zoning ordinance itself picks when it needs a worked example of lot area per unit (§17-17-0304).

How many units can you build on a 3,125 sq ft lot in Chicago?

It depends entirely on the district: RS-3 allows 1, RT-3.5 allows 2, RT-4 allows 3, RM-4.5 allows 4, RM-5 allows 7, and RM-6.5 allows 10. Divide the lot area by the district’s minimum lot area per unit and round down.

Can you build on a lot that is smaller than the district minimum?

Usually yes. A lot lawfully platted before the standard applied is a nonconforming lot, and in a residential district it may be developed with a detached house even though it fails the minimum lot area (§17-15-0202-A). It still has to meet every other bulk standard — setbacks, height, floor area (§17-15-0203). This is why standard lots exist all over RS-1 and RS-2, where the minimum is 6,250 and 5,000 square feet.

How big a house can you build on a 25 by 125 lot?

In RS-3, floor area is capped at 2,813 square feet (FAR 0.9), and the required yards leave a buildable area of 20 by 70 feet — 1,400 square feet per floor. Two full floors plus a partial third is the usual shape, under a 30 ft height limit.

Is every Chicago lot 25 by 125?

No. It is the most common size, not a rule. Corner lots, lots on angled or diagonal streets, consolidated lots and subdivided lots all depart from it, and some neighborhoods were platted to other dimensions entirely. Check the recorded dimensions of the specific parcel before relying on any figure here.

Does a wider lot let you build more?

For floor area and units, what matters is lot area, so a 30 by 125 lot gets proportionally more of both. Width matters separately for side setbacks, which are a percentage of lot width in residential districts, and for the parking exemption that applies only to lots 33 feet wide or less.

Keep reading

For a real address

Check the real numbers for a real parcel

LotGrade reads these rules against a real Chicago parcel — its recorded dimensions, its district, its overlays and the envelope they leave — instead of a standard lot.

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