A 74-unit apartment development in Sandy Hook is under construction.  The complex, called Farrell Communities, is being constructed off Washington Avenue, along a wooded area along the Pootatuck River.  20 percent of the units are being designated as affordable.  Farrell Building Company expects to start renting one- and two-bedroom apartments in two buildings at the end of next month.  Three other buildings are planned at the site.  The property will include 31 detached garages for rent, a clubhouse with meeting rooms, and a dog park.