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The Ministry of Housing, Communities and Local Government authorises competent person schemes, and GOV.UK's published list names FENSA and CERTASS for replacement windows, doors, roof windows or rooflights in existing dwellings and in buildings other than dwellings, alongside schemes including Certsure, NAPIT, ASSURE and Blue Flame Certification. Joining one lets a member self-certify that type of building work instead of getting building regulations approval, provided they meet the relevant minimum technical competence requirements.

CERTASS describes itself as a UKAS accredited certification body and a government authorised certification body since 2006, whose members can self-certify installations as an alternative to Building Control. The practical effect for you is the same under either scheme: the installer notifies, and a certificate reaches you afterwards.

Where approval is required and your installer belongs to no scheme, the requirement does not fall away. Somebody still has to apply to a building control body, meaning the council's building control department or a registered private approver, and the inspection becomes your responsibility to arrange. Settle which route you are on before the old frames come out, not after.

Planning permission and building regulations are separate systems, and most window replacements engage only the second. The Planning Portal's position is that you do not usually need to apply for planning permission for the insertion of new windows and doors that are of a similar appearance to those used in the construction of the house, although a new bay window is treated as an extension and may require permission.

Designated areas change the picture. In a conservation area, national park or area of outstanding natural beauty there may be additional restrictions in place, and a council can make an Article 4 direction withdrawing permitted development rights, after which altering the windows needs an application. A listed building requires listed building consent for any significant works, internal or external, and replacing original sashes with modern sealed units is squarely the kind of change that engages it.

One further rule catches people out on extensions and loft conversions rather than straight replacements: an upper-floor window in a side elevation must be obscure-glazed and either non-opening or more than 1.7 metres above the floor of the room it serves.

Misting between the panes is a failed sealed unit: the edge seal has let moisture into the cavity, and in most cases the glass unit can be replaced without touching the frame. That is a repair rather than a replacement, and it is worth asking for it by name before accepting a quotation for a whole elevation.

Condensation on the room side of the glass is a different animal. Government guidance describes condensation damp as moisture generated inside the home cooling and condensing onto colder parts of it, naming window frames among them. Windows that are markedly more airtight than the ones they replaced can make that worse where ventilation is not provided, which is the reason Part F asks for background ventilators.

Draughts at the edge of a closed casement, sashes that stick, and doors needing a lift to lock generally point to dropped hinges, worn gaskets, a distorted frame or a lock out of alignment with its keep. A competent installer should be able to say which of those it is before recommending that anything is thrown away.

File the scheme certificate with the property paperwork the day it arrives. It is the evidence that notifiable work was notified, and it is what a buyer's solicitor asks to see for any windows or doors replaced since the requirement took effect. FENSA's certificate also records that the warranty is insured, which is the part that matters if the installation company later stops trading.

Where a previous owner left no certificate, the position is the one GOV.UK describes for unapproved work generally: you will not have the certificates of compliance you may need when selling. The routes back are to obtain a replacement certificate from the scheme if the installation was registered, or to ask a building control body what it needs in order to regularise the work. Deal with it before the house goes on the market rather than in the middle of a sale.

Beyond the certificate, TrustMark is the government-endorsed quality scheme for work in and around the home, backed by the Department for Energy Security and Net Zero and the Department for Business and Trade, offering a further check on a firm's standards and a route to complain if the installation goes wrong.

Replacement is not automatically the better answer, and in a listed building or a conservation area it may not be available at all. A failed sealed unit, a rotten section of timber, worn gaskets, a dropped hinge or a misaligned lock are all repairs, and a sash window can be overhauled and draught-proofed while keeping its original boxes and glass.

Replacement earns its place when frames are beyond economic repair, when an elevation is being reworked anyway, when the openings themselves need to change, or when the thermal performance of what you have is genuinely the problem you set out to solve. The regulations follow the work either way: a full replacement is notifiable and needs its certificate, while a like-for-like repair generally is not.