Building a conservatory as a DIY job is certainly possible but certain skills will be essential, particularly if plumbing and electrical work are required. If you do not have these skills yourself, you may perhaps have a friend who can help you. If not, you need to budget for professional help.
First you need to decide just what sort of conservatory you wish to have. Available materials are UPVC, aluminium and hardwood. UPVC is easy to maintain and fairly simple to erect; aluminium is not as popular as it once was because it can cause excessive condensation. A hardwood conservatory can be absolutely beautiful, but bear in mind that regular maintenance is essential.
You may not need planning permission to build your conservatory, but all work will have to comply with building regulations. Make sure that you are familiar with these and that you have the approval of your local building Inspector to carry out the proposed work. If you are putting in a concrete floor you will need to insert a damp proof membrane underneath it, particularly if you are planning on fitting any floor coverings.
You would need to decide whether or not you want to build a dwarf wall and what sort of floor you are going to provide.
What will your roof be made of? A tiled roof is often impractical because of the weight problems but polycarbonate sheeting is a popular option, being both lightweight and relatively inexpensive.
Bear in mind the security aspect. A conservatory is usually a lot less secure than the rest of your house. You may wish to retain a lockable door to protect access to your house from the conservatory.
The best position for a conservatory is usually facing south, but bear in mind that it can get very hot at times so do budget for adequate blinds.
Double glazing for a conservatory is a must. Firstly it is more secure than single glazing, and secondly it will cut down on your heating bills considerably. During winter a single glazed conservatory could get extremely cold leading to possible problems with frozen pipes.
Make sure you buy the conservatory from a reputable supplier, and remember that you will also need to budget for the many other fittings such as facia boards, beading, architraves, tiles, sealants, screws et cetera.
Finally, make sure that you have sufficient, good quality tools available; buying poor quality ones is false economy and could lead to expensive delays in completing your project.