It is easy to assume that replacement plastic windows are maintenance free, however this is not the case. That said, the days of old ungalvanized steel (crittall) windows and draughty timber frames are mainly past.
Modern replacement windows are constructed in a upvc frame. Upvc stands for plasticized polyvinyl chloride which to ordinary mortals like you and I is ‘plastic’. Windows constructed in this material require minimal maintenance but are not maintenance free.
In urban locations the Upvc can discolour and periodically the white plastic parts should be cleansed with a upvc cleaner. Care should be taken not to abrade the plastic surface…………. No scouring pads or ‘Jiff’ or steel wool.
All moving parts should be lightly oiled. In the absence of lubrication exposed south and west facing windows may become jammed and undue force to open them could cause expensive damage to hinges.
Any silicone exterior sealant around the edges of the plastic where it meets the adjacent walls should remain pliant and not be cracked.
Any glazed panels which have misted up should be replaced……… This is probably a job for a professional glazier. The misting is caused by condensation forming between the inner and outer pains of glass where the original vacuum has broken down.
The opening casements are lined with a rubber draught proofing strip. On exposed elevations this rubber can dis-disintegrate and require renewal. In my house the rubbers have lasted 10 years.
You should ensure that all windows can be easily locked, and that the window keys are close by for use in the event of an emergency.
You should plan an escape route from the upper floors in the event of fire. The building Inspector may specify special wide opening escape windows.
These days windows have to be approved by either the Building Inspector, or member of a ‘competent Person Scheme’ the most common of which is either FENSA (fenestration self-assessment, or Certass Ltd). Without appropriate documentation, there may be problems when the property is sold.