Approved Document F is the statutory guidance supporting Part F of the Building Regulations, which governs ventilation in buildings in England. For dwellings it sets minimum standards for background ventilation (a continuous trickle of outdoor air), extract ventilation (in kitchens, bathrooms and utility rooms), and purge ventilation (rapid airing, usually by opening a window).
The version in force from 15 June 2022 tightened the rules for replacement windows and doors: the work must not make a home's ventilation worse than before. Where the original windows had trickle vents or other background ventilation openings, the replacements must provide at least equivalent ventilation.
For landlords this closes a common route by which a window upgrade quietly causes condensation. Replacement window work is notifiable, and most installers self-certify through a competent person scheme such as FENSA or CERTASS — keep the certificate. See the guide on trickle vents and Approved Document F.