A zoning commission's condition that there be an open space may be inconsistent with a zoning requirement that there be an unobstructed legal accessway. In 2008, Michael Innaurato applied to subdivide his 6.39-acre property into three lots. Motor-vehicle access to Lot B-2 would be via a common driveway on Lot B-1. The defendant planning board approved with conditions, one of which was an open space. The plaintiff neighbors appealed, claiming the subdivision violated the zoning regulations, because lot B-2, as an accessway lot, was not served by an unobstructed legal accessway, and because lot B-1 did not meet minimum frontage requirements. The trial court found that substantial evidence failed to support the planning board's decision. Innaurato and the planning board appealed. The Appellate Court held that substantial evidence in the record failed to establish Lot B-2 complied with zoning regulations. Zoning regulation §7(O) provides, "Accessway lots . . . shall be permitted . . . provided that each such accessway lot has access to a street by means of an unobstructed legal accessway held in the same ownership as the accessway lot. . . ." The term "unobstructed legal accessway" is not defined in the regulations, and the planning board's construction was not entitled to special deference. "Accessway," in the context of "unobstructed legal accessway," refers to the part of the accessway lot that connects the interior portion of the property with the street. The planning board required a 2.8-acre open space, and that requirement that the accessway to lot B-2 be kept in a natural state constituted an encumbrance that obstructed the use and development of the accessway. Although a common driveway provided actual access to lot B-2, that did not eliminate the requirement of an "unobstructed legal accessway," under §7(O). The planning board's condition obstructed the accessway and violated §7(O). Also, Lot B-1 could not measure frontage along the private lane, because it lacked the right to use the private lane. The record failed to establish that Lot B-1 met frontage requirements. The trial court properly sustained the plaintiff neighbors' appeal, and the Appellate Court affirmed the judgment.