Owning your own home

I might have found a house for sale (freehold) on a piece of land that I think lends itself well to a knock down and rebuild. At the moment, there exists planning permission for a large extension to the existing building. Am I crazy to think that it might not therefore be too far fetched to imagine that it would be possible to obtain permission to build an alternative house on the same site?

Subsiduary topic - has anyone any experience of self build?

Write to the local authority's planning dept. with an informal enquiry (you could phone, but councils like paper and tend to have formal response time targets for written correspondence). They should be able to give you some advice and hints about what they would and wouldn't approve. Doesn't hurt to ask.