Which era is your house?
The town's own history decides most of the choice. The Victoria County History of Sussex records Horsham's urban district growing from 16,682 people in 1951 to 26,446 in 1971, and the town with Roffey and Broadbridge Heath reaching 38,565 by 1981. Each wave of building used a different window, and the replacement that looks right follows from it.
Read the full guide to Horsham windows by house era, with what was originally fitted in each period and what replaces it well.
Most Horsham windows are on their second round
North Heath was built up extensively in the 1970s. Land at Hills Farm was sold for housing in 1972 and was still being built on in 1980. Southwater, just to the south, had about 1,600 houses by 1982. Houses from those years either came with early double glazing or had it fitted soon after, and a great many of those frames are now forty years old. Misted units, dropped sashes, yellowed white plastic and hinges that no longer hold the window shut are the usual reasons people in these streets get in touch.
Replacing those frames is not the same job as replacing single glazing was. The new windows have to meet current Building Regulations for heat loss and, on upper floors, for escape. The installer handles that through FENSA self-certification, and you get a certificate you will need when you sell. What changes when you replace old double glazing.
The centre is a different matter
Four of the district's conservation areas are in the town itself: Horsham (the Causeway, Carfax and the old centre, designated in 1974), London Road, Richmond Road, and Park Terrace Gardens. The council's appraisals for these areas single out plastic replacements for timber sashes and casements as the main harm to their character. There is also an Article 4 direction on 11, 13, 15 and 19 Denne Road that removes the right to alter windows. If your house is in one of these places, start with the conservation areas page before anything else.
What jobs like these cost
Prices on this site are shown only as whole-job examples built around Horsham house types, from two bedroom windows in a North Heath terrace to a whole 1970s detached house in slimline aluminium. See the example prices.