Sun room, garden room, summer room and conservatory are marketing words rather than technical ones, and the thing that actually separates the products is how much of the envelope is glass and whether the space is thermally separated from the house. Get that distinction right and the comfort, the running cost and the consents all follow from it.
Sun room, summer room extension or conservatory: the distinction that matters
A traditional conservatory is mostly glass, has a glazed roof and is separated from the house by external quality doors, which is what lets it sit outside the normal insulation requirements. A sun room usually has a solid insulated roof, more wall and large windows, and is built and insulated as part of the house. The first is a seasonal room; the second is a room you use in February. Which one you are being sold is a question about the roof and the doors, not the brochure.
Comfort through a British year in a modern sunroom extension
A fully glazed roof is hot in July and cold in January, which is why so many conservatories end up as storage. A solid or largely solid roof with rooflights changes the space completely, and replacing a glazed conservatory roof with an insulated one is a common retrofit for exactly that reason. Ventilation at both ends and some shading do the rest, and both are cheaper designed in than added later.
Consent, and the separation rule
Conservatories are normally permitted development within the same size limits as any other single storey extension, and are often exempt from the full building regulations regime where they are at ground level, under a floor area threshold, largely glazed and thermally separated from the house. Remove the separating doors or build a solid roof and the exemption goes, and the space has to meet the insulation standard like any other room. That is not a loophole to exploit, it is the line that decides which product you have.
Garden extension rooms: building off the house, or in the garden
A detached garden room keeps the house untouched, needs no opening cut in a wall, and under outbuilding rules is often permitted development. It is also separate: you cross the garden in the rain, and it is an outbuilding rather than habitable accommodation unless it is designed and approved as such. For a study, a gym or a workshop that is usually fine. For a dining room it usually is not.
Questions people ask about sun room extension
Is a sun room warmer than a conservatory?
Normally yes, because the roof is insulated and the space is built into the thermal envelope of the house rather than separated from it.
Can I replace a conservatory roof with a solid one?
It is common, and it changes the regulatory position: the structure has to carry the extra load and the space may no longer qualify for the conservatory exemption.
Do sunroom extensions need planning permission?
It is judged as a single storey extension on size, height and position, so many are permitted development. Flats are excluded and designated areas are tighter.
Is a garden room cheaper than extending?
Usually, because there is no groundwork against the house and no opening to form. It buys separate space rather than more house.