"Certificates of assurance” might sound like dry legal documents, but they could generate windfalls for D.C. landlords or perhaps even handcuff lawmakers if they ever try to expand rent control protections in the city.

Its technical-sounding name aside, the principle behind these certificates is relatively simple: if D.C. expands its definition of which buildings fall under rent control, the owners of any affected properties can ask for a tax credit to offset that change….