A new extension holds a great deal of construction moisture in concrete, blockwork and plaster, and takes months to release it. Condensation that continues past that period usually points at ventilation, glazing or a cold bridge.
Drying out
Expect elevated humidity in the first months. Ventilating and heating steadily, rather than sealing the room up, helps it dry without cracking finishes.
Cold bridging at junctions
Where insulation is interrupted — at the wall-to-roof junction, around openings, or at the threshold — surfaces run colder and attract condensation locally.
Glazing and rooflights
Large glazed areas are the coldest surfaces in the room and will show condensation first, particularly on still, cold nights with high internal humidity.
Ventilation strategy
Trickle vents, extract in wet rooms and a habit of purge ventilation matter far more in an airtight new build-up than they did in the draughty original house.