Airtightness limits unintended airflow through joints and connections in the building envelope. It depends on coordinated layers and details rather than visible sealant alone. Planned ventilation remains a separate design requirement for the occupied building.
How this term relates to your project
Roof, wall, floor and frame transitions deserve attention in an extension. Ask where the airtight line runs and how service penetrations are incorporated. A neat internal edge does not demonstrate that the concealed junction has been fully resolved. Establish suitable checks before hidden components are closed.
Reducing draughts is not the same as blocking intended ventilation openings, which have their own role in supply and extraction. Include changes to windows and room arrangements in the ventilation proposal. Photographs and detail drawings help explain the installed connections later, particularly where a future alteration passes through layers that are no longer visible.