Chapter 40A is hereby further amended by inserting after section 3 the following section:-

   Section 3A.

(1) A zoning ordinance or bylaw that permits open space residential development by right or by special permit shall:

(a)  permit the development of new dwellings at least equal to the number allowed under a conventional subdivision plan.  In order to confirm the accuracy of such number a municipality may require either a sketch plan showing the layout under a conventional subdivision scheme or a calculation that deducts for roadways, wetlands and other site or legal constraints and divides by an underlying lot area requirement in order to determine the allowed housing units in the development, but may not require a preliminary design or engineering tests to prove the yield from a conventional subdivision on the property. 

(b) require the proposed development to identify the significant natural and cultural features of the land and concentrate development by use of reduced dimensional requirements to preserve those features.

(c)  require the development to permanently preserve a certain percentage of substantially contiguous developable land, ranging from 30 to 60 percent, in a natural, scenic or open condition, or in agricultural, forestry, or passive outdoor recreational use.  For the purposes of calculating the percentage of land to be preserved, the land’s developable area shall be determined pursuant to applicable state and local land use and environmental laws and regulations, and the zoning ordinance or by-law, without regard in either case to the suitability of soils or groundwater for on-site wastewater disposal as such is separately regulated by local boards of health.

(2) If a zoning ordinance or bylaw contains no provisions permitting open space residential development, then a proposed open space residential development of five or more new single family residential dwellings on a parcel as a subdivision under Chapter 41 in a zoning district that requires a minimum lot area of 30,000 square feet or greater for a single-family residential dwelling shall be allowed as of right if it meets the requirements of this sub-section, except upon a specific finding by the planning board that such development is not feasible or the land and natural resource conservation objectives of such development are achieved on the site through alternate means already adopted by the municipality, such as the transfer of development rights or natural resource protection zoning.  Such developments shall meet the requirements of paragraphs (a) and (b) of sub-section (1) and shall further permanently preserve at least 40 percent of the parcel’s substantially contiguous developable area.  In districts where Title 5 of the Environmental Code is in effect, and which are in nitrogen-sensitive areas where the number of bedrooms is calculated at one bedroom per 10,000 square feet of land area, the provisions of this section shall not apply if the required lot area is 30,000 square feet or less, unless the local board of health approves an aggregate calculation of land area that includes the preserved land, and if the required lot area is more than 30,000 square feet, the minimum preservation requirement set forth in this section shall be modified to equal the percentage resulting from: the subtraction of 30,000 square feet from the lot size requirement: that difference divided by the lot size requirement: and multiplied by 100, except to the extent inconsistent with requirements adopted by a regional planning agency under chapter 716 of the Acts of 1989 or chapter 831 of the Acts of 1977, as those acts may be amended.  A proposed development meeting the requirements of this sub-section shall be permitted upon review and approval by a planning board pursuant to section 81K to 81GG, inclusive, of chapter 41 and in accordance with a planning board’s rules and regulations governing subdivision control. 

Allowance of open space residential development by right in accordance with this sub-section shall not preclude increases in the permissible number of dwelling units within an open space residential development by special permit or otherwise.