This is a question that I used to be asked constantly, frequently by email. As the answer is not straightforward and what is more, I do not do installations like this, I will go into it a little. There are not many cars now that do not have AC fitted as standard and those that do not would normally tend to be from a few years ago. Although originally written perhaps 10 years ago this page may still be slightly useful.
If your car does not have AC at all, apart from a little advice, I cannot help. As to the cost the following paragraphs may help a little. Firstly it should be noted that to install AC into a car whilst it is being assembled is a great deal easier and thus cheaper than it is to add it later to a finished car. Having AC fitted later can be a bit of a shock financially.
Cars in the first category are frequently relatively simple to retrofit AC as the Japanese tend to install the evaporator under the dashboard in front of the passengers knees, where it is relatively easy to access.
The second, European type cars generally have the evaporator mounted underneath or behind the heater matrix right deep behind the dashboard, which means that to access it the whole dashboard with all the wiring, airbags, possibly the steering column needs to come out. This can be a very lengthy and expensive task, possibly up to two days work.
The evaporator is not the whole story of course, many cars have the wiring loom for AC installed in all models, whether actually fitted with AC or not. Naturally this simplifies any later retrofit. Also have a look under the bonnet. Is there room for a mechanic to get his hands into the space around the engine like a Honda Civic or is the under-bonnet space crammed with components as are so many cars where a large or powerful engine is shoehorned into a tiny space, necessitating removal of some other components first in order to mount the compressor (about the size of a kilo bag of sugar) and the condenser (a radiator, similar in size and shape to the water radiator and mounted directly in front of it).
General rule: the more room under the bonnet, the easier and thus cheaper it will be to retrofit.
Type of installation - 4 possibilities
1. Manufacturers kit - usually the best but most expensive option. For Type 1 cars kit cost £600 to £900 with installation and charging cost of say £350 to £500 - say a minimum of £950. For Type 2 cars kit cost £700 to £1200 with installation and charging cost of £500 to £800 - say a minimum of £1200.
2. Approved alternative kit - these kits (Diavia or Eberspacher are very good examples) are made specifically for many models and often use switches similar to the manufacturers own, and utilise good quality components. These are installed only by registered installers who can be found in your local Thomson or Yellow Pages (or their Web equivalents) under the general heading for Air Conditioning Equipment or Car Heating and Air Conditioning. Prices would range from about £1000 up.
3. An installation by an AC technician - varies from excellent to very mediocre. Some I have seen are every bit as good as a manufacturers installation but some have been barely adequate. Fortunately some of the mediocre installers eventually go to the wall. Prices probably from £850.
4. A secondhand system. A possible if the car is not in the first flush of youth and you may not consider it worth spending around £1000 on it. Let's imagine a three year old Mondeo or say a six year old Jaguar. In both of these cases it may be worthwhile to look in Motor Trader or Exchange & Mart at the section following the 'cars for sale' of your make. This usually contains adverts of spares for this make and also details of cars written off.
A car which has been rolled will frequently be written off by the insurance company although the level of damage to the bulk of the car may be minimal. A car like this will probably still be capable of having the engine running and of ascertaining that the AC is functioning correctly or at least is still charged with refrigerant. In these circumstances the AC system may be easily transferable to another car fairly cheaply. The firm advertising the spares may be able to install them in your own car or be able to suggest someone who could.
always recommend that you discard the secondhand drier and install a new
drier from an AC technician (£20 to £40) who could then charge
and test the installed system.