Zoning fundamentalsExplainer

How many units can I build?

Zoning density caps how many homes a lot may hold. Chicago does it with one number, a minimum amount of lot area for every dwelling unit. Other cities cap units in other ways, or not at all.

How Chicago counts units

unitslot arealot area per unit

rounded down to a whole unit

Worked example: the standard 25 × 125 ft Chicago lot, 3,125 sq ft. The method and figures are Chicago’s, from the same rule table the LotGrade analyzer uses, with each rule linked to Title 17 in our hosted zoning code. The math is different in other cities; see the “Elsewhere” note.

Definition

What density means in zoning

Density is the number of dwelling units allowed per unit of land. Chicago expresses it as the minimum lot area each unit needs. Other codes use units per acre, derive a unit cap from floor area, count bedrooms, set a flat cap per lot, or have no unit cap in some districts.

A dwelling unit is independent living quarters for a single household §17-17-0248. A second kitchen can make a building count as two units unless it is clearly accessory, which is why an in-law suite with its own stove draws scrutiny.

Method

Calculating the unit count

Lot area25 ft × 125 ft
3,125 sq ft
Minimum lot area per unitRT-4 — §17-2-0303-A
1,000 sq ft
Lot area ÷ per-unit area3,125 ÷ 1,000
3.125
Units allowedround down — §17-17-0304
3 units

This is the ordinance’s own example: a 1,000 sq ft standard on a 3,125 sq ft lot allows a maximum of 3 dwelling units §17-17-0304. The same lot in three districts:

RS-3

Detached house

The same 25 by 125 foot lot in RS-3: 3,125 square feet divided by 2,500 square feet per unit allows 1 dwelling unit, drawn as a 2-story house.

3,125 ÷ 2,500 = 1.25, so 1 unit.

RT-4

Three-flat

The same 25 by 125 foot lot in RT-4: 3,125 square feet divided by 1,000 square feet per unit allows 3 dwelling units, drawn as 3 blocks stacked in 3 floors.

3,125 ÷ 1,000 = 3.13, so 3 units.

RM-5

Small apartment building

The same 25 by 125 foot lot in RM-5: 3,125 square feet divided by 400 square feet per unit allows 7 dwelling units, drawn as 7 blocks stacked in 4 floors.

3,125 ÷ 400 = 7.81, so 7 units.

Each block is one dwelling unit. The layouts are illustrative; the counts are the maximum the lot-area standard allows before FAR, height and parking. Heights are drawn exaggerated ×1.6.

A ceiling, not a promise

Chicago says so outright: the standards “are not to be interpreted as a guarantee that allowed densities can be achieved on every lot” §17-2-0303-A. Parking, height, unit sizes and the shape of the lot often cap the count first. On the RM-5 lot above, FAR 2.0 allows 6,250 sq ft, which is about 893 sq ft per unit before stairs and walls.

Unit types & exemptions

Smaller units, and units that don’t count

Higher-density districts allow efficiency units (up to 700 sq ft, one principal room §17-17-0249) and SRO units at a smaller lot area each. Efficiencies are capped as a share of the building: 20% in RT-4, RM-4.5 and RM-5; 25% in RM-5.5; 30% in RM-6; 40% in RM-6.5 §17-2-0313-A.

Chicago exemptions that add a unit

  • RS-3 two-flats. The standard drops to 1,500 sq ft, for a two-unit building only, when 60% or more of the lots on that side of the block already have multi-unit buildings, or the lot is in a “Predominance of the Block” area §17-2-0303-B §17-7-0591.
  • Ground-floor accessible units. Ground-floor Type A units (other than in detached houses) don’t count in RS-3, RT-3.5 and RT-4 §17-2-0303-B.
  • Conversion units. A residential building in lawful existence for 20+ years may add units equal to 33% of its long-standing units (rounded half up), or one unit if it has only one §17-2-0303-C.
  • Coach houses. Exempt from the lot area per unit standards §17-9-0201-F. Both are covered in Chicago’s ADU guide.

Chicago

Density in Chicago, by district

DistrictDwelling unitEfficiencySROUnits on 25×125
RS-16,250——0
RS-25,000——0
RS-32,500——1
RT-3.51,250——2
RT-41,0001,0005003
RM-4.57007005004
RM-54004002007
RM-5.54004002007
RM-630013513510
RM-6.530013513510

Minimum lot area per unit in square feet, residential districts §17-2-0303-A. The last column applies the standard dwelling-unit figure to a 3,125 sq ft lot.

In business and commercial districts the standard follows the dash number, not the letter: a B3-2 and a C1-2 lot use the same figure §17-3-0402-A.

DashDwelling unitEfficiencySROUnits on 25×125
Dash 12,5002,500NaN1
Dash 1.51,3501,350NaN2
Dash 21,0007007003
Dash 34003002007
Dash 520013510015

B and C districts, minimum lot area per unit in square feet §17-3-0402-A. SRO units are not allowed in dash 1 and 1.5.

Transit-served B-3 and C-3 projects near CTA or Metra rail, or on listed bus corridors, can use lower figures through a Type 1 rezoning or a planned development §17-3-0402-B. The Zoning Field Guide lists every district’s figures.

Questions

Common questions

How do I calculate how many units I can build?

It depends on the city. In Chicago, divide the lot area by the district’s minimum lot area per unit and drop any fraction. Chicago’s own example: a 3,125 sq ft lot with a 1,000 sq ft standard allows 3 units, not 3.125 (§17-17-0304). Then check that FAR, height, setbacks and parking actually leave room for that many.

Do you round up the number of units?

Not in Chicago: the standard is a minimum amount of lot area for each unit, so every unit needs its full share, and 3.9 units’ worth of lot area allows 3. Rounding rules differ elsewhere — New York City counts a remainder of 0.75 or more as a unit, Seattle only a remainder above 0.85, and California’s density bonus rounds fractions up.

How many units can I build on a standard Chicago lot?

On a 25 × 125 ft (3,125 sq ft) lot: 1 in RS-3, 3 in RT-4, 7 in RM-5 — before FAR, height and parking are applied. In RS-3 a two-flat is possible at 1,500 sq ft per unit where the block qualifies (§17-2-0303-B).

Is density the same as FAR?

No. Density caps the number of homes; floor area ratio caps their total size. A district can allow ten units whose combined floor area FAR only permits at a few hundred square feet each, or the reverse.

Do ADUs count toward density?

In Chicago, coach houses and conversion units are exempt from the minimum lot area per unit standards where they are allowed (§17-9-0201-F, §17-9-0131). See coach houses and conversion units.

What is an efficiency unit?

A dwelling unit of no more than 700 sq ft of floor area, essentially one room plus kitchen and bath (§17-17-0249). Some districts allow them at a smaller lot area per unit, but cap their share of a building: 20% in RT-4, RM-4.5 and RM-5, rising to 40% in RM-6.5 (§17-2-0313).

Keep reading

For a real address

Check the real numbers for a real parcel

The unit count on a real lot depends on its exact area and every other limit at once. LotGrade computes the lot area from parcel data for any Chicago address, applies the district’s density, FAR and height together, and flags ADU eligibility.

Analyze a Chicago address Book a walkthrough

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