Conservatory to extension: convert it or replace it?

A lightweight insulated roof, tiled or flat, replaces the glass or polycarbonate. The windows, doors and base stay. It is the quicker job, often done inside a fortnight, and it fixes the two worst faults of an old conservatory: heat pouring out overhead in winter and glare in summer.

It has limits. Once the roof is no longer see-through, the structure stops being an exempt conservatory, so the work needs building regulations approval. Building control will ask whether the existing frames and footings can carry the new roof, which weighs several times what the old one did. Ask whoever quotes to show you that calculation, and do not accept "it will be fine".

The frames remain the weak point for heat. Twenty-year-old double glazing loses far more than a new insulated wall, so the room gets better, not good. And the doors to the house have to stay unless the whole room can be shown to meet the energy rules, which with old frames it rarely can.

This is the route to choose if you want the room open to the house, on the central heating and counted as part of the home. Everything above the ground goes. What goes back is an insulated floor, cavity or timber-frame walls, a solid roof and new glazing.

The question that decides the cost is the footing. Many conservatories sit on a shallow concrete base, sometimes only 300 to 450 mm deep. An extension normally needs foundations around a metre deep in firm ground and deeper in shrinkable clay or near trees. Before anyone prices the job, have a trial hole dug beside the base so building control can see what is there. If the footing is too shallow, budget for new foundations; the old slab comes out with it.

Low brick walls can sometimes be kept if they are cavity walls on a sound footing. Single-skin dwarf walls cannot.

If the conservatory was built under permitted development, an extension on the same footprint usually qualifies too, since the size limits are the same. One condition changes. Conservatories are excused from the rule that outside materials must look like the house; extensions are not. Brick to match, or render where the house is rendered, keeps you inside it. Cedar cladding on a brick semi may not.

Check the paperwork from when the conservatory went up. Some estates have a planning condition that removed permitted development rights, in which case any replacement needs an application. In Scotland the classes differ and a building warrant is needed for the new structure.

Keep the opening into the house if it already has a proper lintel, the patio levels if they work, and any drain runs that are in the right place. Let go of the floor slab unless it is insulated to current standards, which an older slab rarely is. Let go of the idea of reusing the frames in the new walls: the sizes will not suit and the glass will be the worst-performing thing in the room.