The answer to this question depends entirely on the features you choose to include in your conservatory build.
If you do intend to use your conservatory throughout all the seasons the most important things to consider are 1) how can you insulate it to make it useable in the Winter and 2) how can you keep it cool in the Summer.
Taking each point in turn there are various ways in which your design choices can make or break the usability of the room you are building:
Insulation in Winter.
The features you need to build in to your design include, adding at least 50mm of insulation into the base of the conservatory floor; adding at least 50mm of insulation into any brick walled areas; using A-rated windows in the sides of the conservatory to retain as much heat as possible inside the room; using a good quality insulating glass unit in the roof. And finally you do need to add at least one heat source into the room. Underfloor heating is a good, non-intrusive option – but you may find that you also need to add a secondary source of heating too. You may consider adding an electric radiator or even extending your home gas fired central heating into the new room too. This is allowable as long as the radiator can be isolated from the main home heating.
If you use a good mix of these insulating and heating features you will find that your conservatory is a room you can comfortably enjoy throughout the Winter.
Cooler in the Summer.
The main area you need to address to achieve this is to concentrate on the roof. The roof design is a crucial component in this respect because its location & positioning dictates that it bears the brunt of the heat from the Sun. It is advisable to invest in a high quality glass solution for the roof which is designed to reflect as much of the Sun’s heat as possible. This is achieved by the coating that is placed on the face of the glass. The coating also cuts out much of the glare from sunlight that can make it uncomfortable to be inside the room.
The ultimate way to ensure that a conservatory is usable all year round is to consider adding a ‘solid roof’ instead of a glazed one. The addition of a solid roof is by far the most effective way of blocking out the effects of the Sun and also to provide the maximum levels of insulation possible.