What would you pay for a gorgeous garden? A beautiful, mature plot might not command the premium you expect

A friend's mother always insists that she lives in a garden with a cottage. After years of army postings during which time she satisfied her love of plants by sketching strange and exotic flowers, she finally settled in a sheltered Somerset valley. Nothing will uproot her now from the garden it took so long to create. But for many people, moving is a chance to start again, to get it right. At last the problem of a garden too large or too small or too windy can be remedied. And even though buyers with soil testers rather than tape measures are thin on the ground, gardens can make - or break - a sale.

What is surprising, perhaps, is that a beautiful, mature garden does not necessarily lead to a rush of offers. When Wendy Lauderdale put her three-bedroom Wiltshire cottage on the market in the early summer, she anticipated a quick sale. It was not just that it was pretty, thatched and in open countryside close to the famous Stourhead Estate, but because the garden is gorgeous. Mrs Lauderdale opened it under the National Gardens Scheme and two years ago it was voted as one of the places visitors most enjoyed.

It is the kind of garden most of us can only dream of creating. At the moment it is at a high-summer peak: hydrangeas and the tall, ethereal, lilac-coloured Thalictrum are in full bloom in borders mixed with different shades of bergamot and interspersed with pots of lilies. Alongside a pergola of honeysuckle, roses and clematis - now past its best - Japanese anemones are springing up. Phlox are on the verge of spectacular. Dramatic but also, it seems, daunting.

"So many people say they could never manage the upkeep," said Mrs Lauderdale. "In fact the hard work is done, all it needs is a bit of thinning out, dead heading in summer and then pruning. But I haven't seen a real gardener yet. You would be amazed how many people trip down the path without even a glance at the garden."

It was 12 years ago that she and her husband bought the cottage, on a National Trust lease. The Lauderdales carved the garden out of field and thistle into distinctive areas - which makes it seems larger than its half an acre. Wendy Lauderdale has even written a book, describing its creation. Nevertheless, she is pragmatic about her imminent departure: "I can always create another garden, and I always tell people that they don't have to keep it as it is. They can concrete it over if they want to."

Size of garden can prove a sticking point, estate agents find. Simon Barker of Michael de Pelet who, with Knight Frank, is selling the Lauderdale's house for offers in excess of pounds 185,000, said that the Lauderdale's garden has helped the price. But he added that people interested in a cottage would not be keen to employ a gardener, whereas this would be taken for granted with a large house and garden. Nor do buyers stipulate what kind of garden they want. "Occasionally we get people asking for walled gardens, but not many are specific - it limits their choice of house," he said.

Many buyers who are keen gardeners like the idea of starting from scratch and the potential of a house with either a neglected garden or surrounded by rough land has a strong appeal, especially if it is in a good position. While at the other end of the scale, there is a premium on houses with gardens designed by someone famous. Ian Stewart of Savills reckons that a Gertrude Jekyll garden, for instance, adds at least 10 per cent to the value. "It has to go to the right person, someone who appreciates the planting and that a good garden cannot be created overnight."

Certainly, in London, where designers are at work on pea-sized patios, an established garden with traits of Jekyll-inspired naturalism would be snapped up. But, again, size can be a drawback. Ben Stagg, of Goldschmidt & Howland's Hampstead office, says that some people refuse to touch a large garden with a bargepole, even if the upkeep is less than for a small, intricate plot. "If a family does want a large garden, they often go for zero maintenance - trees and a good area of grass for the kids to play on." A good-sized period property in Hampstead with a garden of about 80ft would be in the region of pounds 800,000 to pounds 1m.

As for the perennial search for the perfect south-facing plot, in many cases this is a waste of effort, says Mr Stagg. "A south-facing garden is no good if you have very tall building or trees at the end. A north- facing garden that is open to the west can be much sunnier."

However, in central London, the best kept secrets are the secluded garden squares, hidden from prying eyes. No one who spends just part of the year in town wants the burden of a garden. Buy a house in, say, Green Street in Mayfair (about pounds 2m), says Simon Barnes of Lassmans, and with it comes access to a communal garden with fountain, flower beds and privacy. And if you want to party, sur l'herbe, there are no worries about weeds. The Grosvenor Estate sees to all that.