522.1 If land disturbance associated with a major substantial improvement activity constitutes a major land-disturbing activity or is part of a common plan of development with a major land-disturbing activity, then the property owner shall comply with the performance requirements for a major land-disturbing activity pursuant to § 520; otherwise, the property owner shall comply with the provisions of this section.

522.2 For the purposes of calculating the cost of a major substantial improvement to a building or structure, an applicant may exclude the cost of replacing manufacturing and industrial equipment, including pumps, valve chambers, and wastewater treatment facilities, but may not exclude the cost of replacing boilers, furnaces, and other equipment that is part of the heating and cooling system or other infrastructure commonly found in a building or structure.

522.3 For a site that undergoes a major substantial improvement activity, the property owner shall employ and maintain each Best Management Practice (BMP) and land cover necessary to comply with the requirements of this section until site redevelopment that follows a Department-approved Stormwater Management Plan (SWMP) occurs.

522.4 For a site that undergoes a major substantial improvement activity, the property owner shall retain the rainfall from an eight-tenths (.8) of an inch rainfall event, which is the eightieth (80th) percentile rainfall event for the District of Columbia, measured for a twenty-four (24) hour storm with a seventy-two (72) hour antecedent dry period by:

(a) Employing each BMP necessary to retain the eight-tenths (.8) of an inch Stormwater Retention Volume (SWRv) requirement, calculated as follows:

$$\text{SWRv} = P \times [(Rv_N \times N) + (Rv_C \times C) + (Rv_I \times I) + (Rv_B \times B)] \times 7.48 / 12$$

where:

The surface area shall include the combined total of the substantially improved building footprint and the land disturbance,

SWRv = volume, in gallons, required to be retained,

P = 80^th percentile rainfall event for the District (.8 inches),

RvN = .00 (runoff coefficient for natural cover),

RvC = .25 (runoff coefficient for compacted cover),

RvI = .95 (runoff coefficient for impervious cover),

RvB = .95 (runoff coefficient for BMP cover),

N = post-development natural cover surface area,

C = post-development compacted cover surface area,

I = post-development impervious cover surface area,

B = post-development BMP cover surface area;

(b) Employing each post-development land cover factored into the SWRv; and

(c) Calculating separately and achieving the SWRv, with P equal to 1.2 inches, for the portion of land-disturbing activity that is in the existing PROW, in compliance with § 521.

522.5 For a site that undergoes a major substantial improvement activity, the property owner may comply with the eight-tenths (.8) of an inch SWRv requirement on-site or through a combination of on-site retention and off-site retention, under the following conditions:

(a) The site shall retain a minimum of fifty percent (50%) of the eight-tenths (.8) of an inch SWRv requirement on-site, calculated for the entire site, unless the Department:

(1) Approves an application for relief from extraordinarily difficult site conditions; or

(2) Provides a waiver, which shall be granted if:

(A) The site drains into the Gray Combined Sewer System (Gray CSS) from a drainage area that is not targeted for sewer separation, as documented in capital improvement budgets; and

(B) The property owner signs an agreement committing the project to use only High-Impact Stormwater Retention

Credits (High-Impact SRCs) or Low-Impact Voluntary SRCs to comply with its Off-Site Retention Volume (Offv) requirement pursuant to § 527; and

(b) Pursuant to § 527, the property owner shall use off-site retention for the portion of the SWRv requirement that is not retained on-site.

522.6

For a site that undergoes a major substantial improvement activity, the property owner may comply with on-site retention requirements by retaining more than the eight-tenths (.8) of an inch SWRv requirement for an area of the site, subject to the following conditions:

(a) Unless a Site Drainage Area (SDA) drains into the Combined Sewer System (CSS) or the Department approves an application for relief from extraordinarily difficult site conditions, the property owner shall comply with at least fifty percent (50%) of the eight-tenths of an inch (.8 inch) SWRv requirement from the SDA through:

(1) Retention; or

(2) Treatment to remove eighty percent (80%) of total suspended solids;

(b) Unless an SDA drains into the CSS or the Department approves an application for relief from extraordinarily difficult site conditions, the entirety of an area intended for use or storage of motor vehicles shall drain to each necessary BMP to comply with at least fifty percent (50%) of the eight-tenths (.8) of an inch SWRv requirement flowing from that entire area through retention or treatment;

(c) Retention in excess of an eight-tenths (.8) of an inch SWRv requirement for one (1) area of the site may be applied to the volume required for another area of the site;

(d) Unless the Department approves an application for relief from extraordinarily difficult site conditions or waives the minimum on-site retention requirement pursuant to § 522.5(a)(2), the property owner shall comply with a minimum of fifty percent (50%) of the eight-tenths (.8) of an inch SWRv requirement for the entire site; and

(e) Retention of volume greater than that from a one and seven-tenths (1.7) inch rainfall event, calculated using the SWRv equation with a P equal to one and seven-tenths (1.7) inches, shall not be counted toward on-site retention.

522.7 A major substantial improvement activity may achieve on-site retention by directly conveying volume from the regulated site to a shared BMP with available retention capacity.

SOURCE: Final Rulemaking published at 35 DCR 21 (January 1, 1988); as amended by Final Rulemaking published at 60 DCR 10640 (July 19, 2013); as amended by Final Rulemaking published at 67 DCR 818 (January 31, 2020); as amended by Final Rulemaking published at 72 DCR 012101 (October 31, 2025).