Ain't the radiators are standard when you hire a builder to construct a conservatory.

Unless the radiators were in the original build specifications, or that you instructed him prior to the construction, then installation of any radiators or electrical points will be an additional item and will need costing accordingly

Hi, generally speaking you would have the discussion of heating with a builder at the beginning of the process however this would depend on how much of the build they were doing. As a conservatory requires it's own heating separate to your existing home heating system they may leave it to you so you can choose a more suitable system for your requirements such as underfloor heating.

If the builder quoted a complete job the radiator should have discussed at the outset of the original quote and itemised within the fixed price . We always discuss electrics and heating on every new build as there an essential element to hand over a complete structure. Maybe the client should also have asked about heating at the off set. However if heating is required and it's a simple tee off existing heating pipes £400-£600 ISA ball park depending on size and heating output requirements.

Unless you have full planning a conservatory should not include radiators attached to the house heating system anyway