For developments or enlargements of buildings, except sky exposure plane buildings, in which at least 50 percent of the dwelling units either comply with the definition of “affordable housing unit” set forth in Section 27-111 (General definitions), or have a legally binding restriction limiting rents to households with incomes at or below 80 percent of the income index, as prescribed by a City, State or Federal agency, law regulation, or regulatory agreement, for a period of not less than 30 years, or at least 50 percent of its total floor area is a long-term care facility or philanthropic or non-profit institution with sleeping accommodation, the Board of Standards and Appeals may modify the underlying bulk regulations, other than floor area ratio or the maximum height of buildings or other structures, and provided that the Board finds that:

1. there are physical conditions, including irregularity, narrowness or shallowness of lot size or shape, or topographical features that create practical difficulties in complying with the bulk regulations and would adversely affect the building configuration or site plan;
2. the practical difficulties of developing on the zoning lot have not been created by the owner or by a predecessor in title;
3. the proposed modifications will not unduly obstruct access of light and air to adjoining properties or streets;
4. the proposed scale and placement of the development or enlargement relates harmoniously with the surrounding area; and
5. the requested modification is the least amount necessary to relieve such practical difficulties.

The Board may prescribe appropriate conditions and safeguards to minimize adverse effects on the character of the surrounding area.