After five years of delays, the first phase of Queset Commons, a large, mixed-use affordable housing development, is set to begin. State legislators approved a law in 2005 to give communities more local control over proposals for dense development, but a weak economy and a cititizens’ appeal of a Conservation Commission determination stalled the project’s progress. This development by Douglas A. King Builders calls for 280 housing units and 116,000 square feet of commercial and retail space on 50 acres at routes 123 and 138. The Planning Board has approved initial work to construct 98 housing units, one-fourth of them affordable, and some retail. Developers, as part of a $2 million-plus mitigation plan, will allocate eight acres to open space; pay to renovate and expand the Depot Street fire station; conduct traffic improvements at intersections; and allocate some capacity at the project’s wastewater-treatment plant for the town’s use.