Amenity land used for grazing sells at about �10k per acre. This is about one fortieth of an acre, so �250 would be an average market price. But critically, the price of land sells at whatever someone is prepared to pay for it and land valuers will try and estimate the potential extra value your house might be worth if it had a bigger garden - probably a couple of thousand if its really tiny at present. You'll have to factor in the costs of solicitors to transfer it.