Maximum Dwelling Units

Support Field

Maximum Dwelling Units (MAXDU)

• Associated Fields: MXDU_SPEC, MXDU_OVE, MXDU_CALC1, MXDU_CALC2, MXDU_CALC3, MXDU_EFF, MXDU_ESVAL

• Value Referenced In: MNLS_CALC, DUpAC_CALC1

This field catalogs the maximum number of dwelling units allowed on a lot by-right.

Interpretation

As with other dimensional fields in the Zoning Atlas, in zones with multiple sets of dimensional regulations, the published value corresponds with the greatest density use allowed in the zone by-right. Greatest density is determined by maximum dwelling units per acre (DUpAC), in the case of residential development, or by the floor-area ratio (FAR), in the case of mixed-use or non-residential development.

MAXDU is not often included in the section of the bylaw or ordinance that specifies dimensional regulations. In these cases, MAPC checked allowable uses in the zone. If the zone did not allow housing by-right, MAPC recorded a MAXDU value of 0; if the zone allowed housing by-right but the maximum was vague – for example, a zone that allowed “Multi-family Residential” or “Apartments” by-right – MAPC checked to see if a value was specified in the definition of that term. If one was not, we left the field blank to indicate that there was no defined maximum.

While most zones allow any number of dwelling units under the defined maximum to be constructed, this is not universally true: some zones also specify, or imply, a minimum number of dwelling units. For this reason, it would be inaccurate to apply the MAXDU field as a range of allowable dwelling units that starts at 1 and ends at the specified value for MAXDU.

Example: In the table below, Principal Use 6.1, Multifamily or apartment housing -- a use with a greater number of allowable units -- is allowed by-right in the DC District, while Principal Uses 6.2, Single-family housing, and 6.3, Two-family housing, are never permitted.

Image 6: Prohibited Lower Density Uses

Estimation

MAXDU is populated for approximately 50% of the zones in the Atlas where residential development is allowed by-right.

Where MAXDU was not specified in the Bylaw or Ordinance, MAPC was able to approximate MAXDU for an additional 30% of zones using three different methods, which all treat MINLOTSIZE as a lot size maximum:

• MXDU_CALC1: If LApDU is greater than 0, maximum dwelling units are estimated by dividing MINLOTSIZE by LApDU and rounding this number down.

• MXDU_CALC2: If MINLOTSIZE and FAR are greater than 0, maximum dwelling units are estimated by calculating the building square footage (MINLOTSIZE * FAR) and dividing this figure by the estimated SFpDU, 1,000SF.

• MXDU_CALC3: If MINLOTSIZE, PCTLOTCOV, and MXFL_EFF are all greater than 0, maximum dwelling units are estimated by calculating building square footage (MINLOTSIZE * PCTLOTCOV * MXFL_EFF) and dividing this figure by the estimated SFpDU, 1,000SF.

Support

MAXDU is used to estimate MINLOTSIZE. The value in the Effective Maximum Dwelling Units (MXDU_EFF) field is used in one estimation of Maximum Dwelling Units per Acre (DUpAC_CALC1).