Restrictive clauses in your lease - You need to check your property deeds for any restrictive covenants. Some developers place a restriction on any building work that will affect the external appearance of the house. If there is such a clause, contact them, as it can often be circumvented for a fee. Your other option is to convert the inside of the garage without changing its external appearance. This is often achieved by keeping a front portion of the garage for storage space and converting the rest. Of course, the suitability of this option depends on your intentions for the garage.
Planning permission - You usually only need to apply for planning permission to convert a garage when you intend to extend the actual size of the garage. However, check with your local authority as the precise rules do differ.
Building Regulations - If you plan to convert any part of your house into a room to be used as habitable space, you will need to comply with government building regulations. A building control officer will probably need to check your conversion a number of times during construction. More information can be found on the relevant section of the government's website. Again, rules differ so you need to check with your local council. There are some instances where permission to convert your garage may be denied such as:
You live in a listed building or neighbourhood - It is very likely in this case that the external appearance of the house must be maintained as it is.
Your garage conversion would affect drainage - If you plan on concreting over your garden or anything similar, thus putting extra pressure on the existing drainage in place, you may need to invest in a solution to allay the potential for flooding or blocked drains.
Additional insulation is needed - Some councils will allow you to simply add the extra fill to the existing construction, others demand that you dig up the floor and insulate it as if building from scratch. This will affect your budget, so always check first.
Parking restrictions - In some cities where there is a particular shortage of parking spaces such as London, councils can refuse permission to convert your garage from a place where a car is kept.