Create I want to Hire an Architect for a Single Storey Extension? Lean-to Back Expansion Expenses Lean-to extensions usually are a single room-sized expansion to the back of home. The roof on these extensions is normally pitched, that is in which the name arises from. A lean-to expansion is a type of selection for little extension. Costs are around A?25,000 – A?40,000. Wrap-a-round Extension Cost A wraparound expansion could be