ADU Construction in St. Louis, MO

St. Louis has a mixed housing stock: 43% detached houses, 4% attached or townhouse units and 26% in buildings of five or more units.¹ On a mixed stock, feasibility turns on the individual lot - detached houses on larger parcels are the usual candidates; attached homes rarely qualify. Building permits in St. Louis are normally issued by the city, which is not part of a county.^2,3 ADU plan review and permits go through the city building department; the contractor prepares the plan set and submits it.

Owners and renters are roughly balanced in St. Louis, with 45% of occupied homes owner-occupied, below the Missouri share of 68%.¹ Where most occupied homes are rented, ADU inquiries often come from owners of rental property; the feasibility review is the same. The stock is old by national standards: 86% of St. Louis's housing units were built before 1980 (U.S. 50%), and only 4% since 2010. The largest single share, 58%, dates from before 1940. That makes it much the same age as St. Louis city as a whole (86% built before 1980).¹ Older lots often have existing detached garages, the usual candidate for a conversion; the slab height and moisture barrier decide the real scope.

St. Louis sits in IECC climate zone 4A (Mixed-Humid).⁴ In this climate zone the energy code sets wall and ceiling insulation levels that a conversion must meet. The median owner-occupied home in St. Louis is valued at $197,500 (ACS), below the Missouri median of $230,300. Within St. Louis city the median is $197,500, so St. Louis sits close to it.¹ With values below the state median, phased scopes - the essential work first - are a common way to keep a project sensible.

Heating dominates the year: about 4,433 heating degree days against 1,713 cooling degree days (base 65°F).^5,6 In a heating-led climate, insulation and air sealing carry most of the energy-code weight for any work that opens the envelope.