Once an extension has been made weather-proof an opening is normally made through the existing external walls.
This can be achieved by removing any existing French doors, patio or window openings and, providing the span of and loading on the existing lintel over the existing doors or windows is not increased, further support is not normally needed.
If a new, or wider, opening is to be formed, the remaining wall above the new opening will need to be supported, typically by installing one, or more usually, two new, properly designed, beams. Any new beam should normally have at least 150mm bearing (overlap onto the existing wall) on each side of the opening and the existing wall beneath the bearings are likely to need to be strengthened to prevent crushing of them. This may require the installation of an area of dense concrete (cast in-situ or pre-cast), known as a padstones to spread the load. The size of padstones will vary depending on the circumstances of the case in hand.
If the beam is steel then it should normally be protected against fire so that it will have 30 minutes resistance to fire (if measured in a standard test). There are different ways that this may be achieved, but the most common is the use of two or more layers of properly fixed plasterboard - the thickness of which will depend on the manufacturer's specification.
If an exposed timber beam is preferred then a calculation is generally required to demonstrate how much inherent fire resistance it has - dependent on its size and species of timber. A concrete beam, which would normally have steel reinforcement inside it, generally has adequate fire resistance properties, providing the steel inside is adequately covered by the concrete.