Housing supply is often discussed as though it were a single quantity. Public datasets usually describe several distinct stages: existing stock, starts, units under construction, completions, approved projects, unsold units, and expected future completions.
These measures can move in different directions at the same time. A market can have strong completions this quarter while its future pipeline contracts. A large pipeline can coexist with tight current availability because the units have not yet entered the usable stock.
Stock
Stock is the inventory that exists at a point in time under a source definition. It may refer to all dwellings, private residential units, purpose-built rental units or another defined universe. A stock measure answers “how much exists?” rather than “how much was added?”
Flow
Starts and completions are flows over a period. They answer questions about movement into development or into completed stock. Comparing a quarterly flow directly with a year-end stock without a common time basis can create misleading ratios.
Pipeline
Pipeline measures describe possible future supply. Depending on the source, they may include projects with planning approval, projects under construction, units expected to complete, or unsold units in approved developments. Each stage carries a different degree of certainty.
A pipeline is therefore not a promise of delivered housing. Timing can change, projects can be revised, and definitions can differ across jurisdictions.
Reading the combination
The most useful supply interpretation often comes from the relationship among measures. Stock growth shows structural expansion. Starts can indicate development response. Completions show what actually entered the completed inventory. Vacancy can indicate whether completed stock is being absorbed. Pipeline measures provide forward context.
None of those measures alone explains “supply.” Together, with their definitions intact, they describe different stages of the same system.