An outside tap is very useful, with the cheap availability of pressure washers on the market outside taps have become very common place and can be readily purchased at DIY stores such as wicks under easy fit external tap kit.
First of all the tap needs locating preferably above a drain / trap so that the water can drain away from the property.
You basically have two choices of tap installations here-
1.purchase an off the shelf kit which comprises of a outside tap, a self cutting tap for inside which clamps to your existing cold water pipe and a flexible hose which connects the two taps together.
2.Purchase a back plate and a outside tap enough copper / plastic pipe and the required amount of joints.
Start by drilling a hole through the wall big enough for a piece of 15 mm pipe or garden hose if using a kit type to pass through, it is best to drill with a small diameter bit first all the way through the wall then drill in from each side with a larger drill bit.
Turn the isolation valve off if there is one on your cold water supply pipe where your new garden tap is to be connected.If there's no valve, turn off your main stopcock. This is fitted on the pipework and normally found either in the kitchen or where the mains water enters your house.
For copper fitting.
Push a length of 15mm copper pipe through the wall and cut another piece of pipe long enough to reach from the pipe shoved through the wall to the approximate position of the tap. Blow any debris out then Join these two pieces together using a elbow, now fasten the Back Plate on to the open piece of pipe making sure every thing is clean, if you are using a compression joint put some PTFE tape around the thread.
Push the back plate against the wall and mark the position of the drill holes, this is easy as the pipe will turn 360 degrees as the pipe is not yet connected inside! Turn the back plate and attached pipe then it is not in your way and drill the three holes, plug the holes and fasten the back plate to the wall with either brass or stainless steel screws as normal screws will rust outside.
Put plenty of PTFE tape around the thread on the Tap and screw it into the Back Plate, hold the back plate with a pair of water pump pliers and screw the tap home.
Break into cold water pipe and fit a T piece connection.
Couple everything up, turn water on and check for leaks.
For kit type.
No need to turn water off as will be supplied with self cutting sealing valve.
Same procedure only using a flexible hose. The difficult part is getting the flexible hose through the drilled hole.
The only tried and tested way I have found is by inserting a reasonably strong wire rod into the hose, secure it with tape making like a cone shape. Negotiate the wire through the pre drilled hole, once wire is through try and manipulate the flexible hose through. Not easy but with a bit patients can be done.
Connect the self cutting valve up and bobs your uncle.
Please make sure any outside taps if left on in winter have suitable lagging or a box fitted.