We use hedges for privacy, to mark boundaries, for their aesthetic value, as windbreaks and for security. Hedges can be useful to reduce traffic noise or provide habitat for wildlife. So why does my heart sink when I get a 'phone call from someone who wants their hedge trimmed because it's got a little out of hand?
You do not need planning consent to plant a hedge (despite the fact that I'm sure some urban hedges can be seen from space) but there may be covenants attached to your property which prevent you from enclosing your garden. If you decide to plant a hedge choose your plants wisely and have some consideration for your neighbours.
There is actualy no law to prevent you from having a six metre high hedge providing no-one complains but for most of us 2 metres is adequate for the purposes of privacy. So why have a hedge? a 2 metre high hedge will be almost a metre wide and so you need to plant it half a metre from your boundary. A 1.8m fence is just 100mm wide so giving you 900mm that you'd otherwise lose. The width of your hedge may cause you problems. If it grows over a public highway or pathway you may be required to cut it back - this is especially true where it prevents the use of mobility devices. If it encroaches into a neighbour's property they may invite you to reduce it - if you decline they may undertake the work themselves and leave the cuttings for you to dispose of.
There are places for hedges in urban gardens, they can be very useful when trained to sit atop a wall to prevent passers by from doing just that but they need regular maintenance. Let's be honest problem hedges (such as Leylandii and Lawson Cypress) have become a high profile issue. Conifers need to be clipped regularly as they rarely survive hard pruning so if you plant a hedge that makes a metre of growth a year be prepared to remove that much vegetation and dispose of it.
The RHS offer comprehensive advice on the selection of plants for hedging as do most specialist nurseries. All advise you to grow thorny plants for security and all fail to mention that this isn't a good idea in gardens where children play.