The kilns of these buildings vary in shape depending on which part of Kent you are in: "In the Weald, you will find small single kilns, which used to be part of a yeoman's farm. Towards the middle of the county, there are larger groups of oast houses on their own and, in East Kent, it is rare to see a roundel. Most of the oasts have square kilns, which are easier to convert," says Simon Backhouse, of Strutt & Parker's Canterbury office.
Many people think that oasts are just the preserve of Kent, but they can also be found in the counties of Sussex, Hampshire, Herefordshire and Worcestershire. However, Kent has always been responsible for a major part of hop production. In 1995 it produced around 46 per cent of the total amount grown.
Hops were introduced to this country in the early 16th century, with oasts being first rectangular in shape and then square in the early 18th century. It was only in the early 1800s that round oasts appeared. As these were replaced by more efficient, modern, barn-like buildings, the old oasts have been converted into attractive houses.
"There are not many that come on the market for conversion now," says Nigel Maclean, of Calcutt Maclean Standen in Wye. "Most of the redundant oasts have already been converted. Some people love the idea of living in one, but others just view them as glorified barns and don't buy them because they do not like barn conversions."
As with any conversion of an old agricultural building, there are restrictions on what you can or cannot do, and the biggest difficulty with oasts is the kiln itself, especially if it is a round one: "The biggest problem is the cost of looking after the roundel roof over the kiln," says Gary Mitchell, a developer who has converted a number of oasts. "Their condition depends on how the farmer looked after them.
"There is a very high cost of re-installing the roof because it is conical, and because the timber work is nearly always in a bad state. This is because of the years of heat, and often dry rot."
And because almost all the roundels are rendered on the inside, it is difficult to estimate its condition until work is started: "One property we were working with was far worse than we expected. On the face of it, it looked OK, but the timber work was completely finished inside," says Mitchell.
Another problem, which is also encountered with barn conversions, is where to put the windows, and how large they can be. Planners demand that windows put into the roundel are as small as possible, so as not to destroy the original outline, but if you have a south-facing oast in a good position, then it can work well and be quite light enough inside. "It all depends on where the oast is located," says Murphy.
The white cowls at the top usually need to be replaced too, and this can be done with glass fibre. But if the building is listed, then the planners will want to see wooden ones put back on, which are expensive. "The cowls have to move, and some people are put off because of the noise they make - they sort of creak," says Simon Backhouse.
The roundels, which can be anything from 16ft to 20ft in diameter, are often used as bedrooms or kitchens, and can make lovely sitting-rooms if they have a southerly aspect. "Purchasers do wonder how they are going to furnish them. Some have curved radiators, but most people find that their ordinary furniture looks fine," says Maclean.
Wendy and Tom Spiegel bought their oast in Goudhurst, Kent in 1996. "We were looking for a house with space and character, and being an oast was an added attraction. It was quite derelict when we bought it and it took a year to convert. We have the study, sitting room and two bedrooms in the roundels, and I never notice that the windows are small.
"We were not allowed to have them facing the road, but have three of them so there is plenty of light." In the study, they had round oak desks built, and all the shelves are curved around the walls.
Calcutt Maclean Standen's Cranbrook office is selling Yew Tree Oast in Hawkhurst. This was converted in the Seventies and has a dining-room and bedroom in the roundel. The white-timbered oast has three bedrooms in all, and there is also a converted barn with two bedrooms. It is on the market for pounds 550,000, and there is a cattery which can be purchased separately.
Also in Hawkhurst is Woodsden Oast. This is a black-timbered oast with five bedrooms and is on the market for pounds 485,000. Strutt & Parker is selling The Oast House in Badlesmere, Kent. This is a brick and flint property with two roundels. There are five bedrooms, along with gardens of 1.85 acres, and the agent is looking for offers in the region of pounds 425,000.
In Littlebourne, near Canterbury, a very fine three-kiln oast is for sale. This has been used as five flats but is being sold for refurbishment and conversion to a single home. It is sandwiched between the Little Stour River and the village green, and has the potential to be turned into a stunning house. GW Finn & Sons is selling the property, with a guide price of pounds 200,000.
Calcutt Maclean Standen, 01580 73250; Strutt & Parker, G.W.Finn & Sons, 01304 612147