The Law

Civil Aviation legislation applicable to kite flying is contained in the  The Air Navigation Order and Regulations - also known as CAP393.   These are amended from time to time but for the purposes of these documents kites are classified as aircraft.  The majority of the rules really apply where the kite weights over 2kg but many apply to all kites.

The CAA can be contacted at:

Airspace Utilisation Section
Directorate of Airspace Policy
Civil Aviation Authority
45-59 Kingsway
London WC2B 6TE
Tel:  01293 567171

The parts that are particularly relevant to most kite flying are:

A kite shall not be flown at a height of more than 30 metres above ground level within the aerodrome traffic zone of a notified aerodrome during the notified operating hours of that aerodrome. [Although it is generally accepted that for safety reasons you should not fly within 5km of any aerodrome].

A kite shall not be flown at a height of more than 60 metres above ground level.

There are additional rules regarding visibility of kites during the day and night where permission has been granted to exceed 60 metres.

For further details of the Act above look at the  For permission to exceed the limits stated above you are required to complete an application form.

Hints and Tips

Flying a kite is easy, but there are a few simple tips that make flying the kite even easier and, more to the point, safer for you and others around you.

Always read the instructions first. These would normally give the proper assembly information as well as basic flying hints and tips. A guideline to the strength of the wind suitable for the kite may also be printed - do not exceed this as the kite can become uncontrollable. See below for a simple wind speed guide. 
Never fly kites in wet or stormy weather. Static electricity can build up and be conducted down the line. This is also the reason why you should never fly a kite with wire or anything metallic in the line.
Never fly kites over other peoples heads or in an area where someone else could be injured from an out of control kite. If you are new to kite flying make sure there is plenty of room around you.
Do not fly close to roads or paths. Not only can it be dangerous if the kite comes down but it can distract drivers as well.
Keep away from overhead power lines, transmission towers, telephone lines and aerials. If your kite gets caught - DO NOT attempt to rescue it yourself - ask for help from the right people such as the electricity company.
Always be aware of what is behind you, be it people, roads or even cliffs! It is easy to be distracted by the kite and step back.
Always wear gloves for strong pulling kites.
Do not fly near airports or above 200 feet (60 metres) - see the Rules of the Air.
Always tidy up after you. Take away any odd bits of line you have discarded, the bag that the kite came in, etc. Throw them away responsibly or recycle them.
Be careful of animals, they can be easily frightened by flying kites - particularly dual and four line kites.
If you are new to kite flying then consider contacting one of the many local groups for advice- see the group directory. If you have purchased a dual line sports or power kite, consider taking lessons in how to fly the kite, the kite trader you bought the kite from should be able to guide you in the right direction. Training is particularly important for power kites as these can be very dangerous if not flown properly.
When flying in the vicinity of horses please be aware of the issues by reading this article.
Finally, be courteous and think of others. Not everyone is happy with kites buzzing around them. If someone else thinks your kite is a danger to others and asks you to stop - do so. They may be more aware of what is happening than you are. 

Fly Safe - Have Fun - you can also find some more tips on safe flying here.

Wind Speed
There is a general scale of wind speed known as the Beaufort Scale. This gives a simple indication of the wind speed which can be matched to the recommended wind speed for the kite you are flying.