People tend to assume that listed buildings are beautiful old ruins and that if you tend them with loving care and elbow grease, they will polish up as a modern pad.

But listing means that if you were to replace so much as a pane of glass without permission you could be fined thousands of pounds.

Listing ensures that the architectural and historic interest of a building is carefully considered before any alterations, either outside or inside, are agreed.

Simon White, surveyor and director for Ashdown Lyons, explains: "The concept of listed buildings was introduced during the Second World War as a way of deciding which buildings should be rebuilt and which should be demolished as a consequence of the Luftwaffe's periodic nightly visits."

Buildings can be listed because of age, rarity, architectural merit, and method of construction. Occasionally English Heritage selects a building because it has played a part in the life of a famous person, or as the scene for an important event.

All buildings built before 1700 which survive in anything like their original condition are listed, as are most of those built between 1700 and 1840.

We've all heard of grading for listed buildings, but what do they mean? Grade I buildings are of exceptional interest, while Grade II buildings are particularly important buildings of more than special interest or of special interest.

Listing currently protects 500,000 or so buildings, of which more than 90 per cent are Grade II. Listed buildings may be eligible for English Heritage grants for urgent major repairs, but if you do have a listed building and it's Grade II, you are very unlikely to get any sort of help.

In fact, Simon White warns that buying any sort of listed building can be a nightmare.

"A majority of people don't know what they are getting themselves into or how to get themselves out when it all goes horribly wrong," he warned.

A friend of his fell in love with a gigantic Georgian house built around the 1750s. And although Simon advised his friend that restoring this building would be costly, he went ahead.

The leaking roof, which was made of Welsh slate, had to be repaired using natural slates, preferably from Wales. And his friend even had to seek planning permission to put in a period fireplace from a space where it was ripped out in the 1960s.

Why? Because the building itself was not listed until after the 1960s, when the fireplace had already been ripped out. This meant any alterations made subsequently to the home needed planning permission.

The moral is: if you must go old, and you must go listed, check out all the pitfalls very carefully and don't let your heart dictate what your brain is screaming out at you at the time. Think it through.