The only thing you'll find online is the title plan (�3) and the title register (also �3) for your property. They don't provide a great deal of detail. For example, this is what a title plan looks like:
http://www.landregisteronline.gov.uk/lro/resou rces/example_title_plan.pdf
and here's an example of a title register:
http://www.landregisteronline.gov.uk/lro/resou rces/example_register.pdf
They're probably of no use to you but, if you're interested in buying them, go here:
http://www.landregisteronline.gov.uk/
If the house has been built (or modified) since the Town and Country Planning Act 1947 came into effect (or even if a planning application was submitted for changes which never came about), your local council will have copies of the planning applications, which will probably include plans which are far more detailed than the title plan. Unless you live in an area covered by a unitary or metropolitan authority, it's the district council (and not the county council) which will hold the planning applications.
Records of planning applications aren't available online but they're freely available to anyone who calls at the relevant council office. (You don't need an appointment and there's no fee to be paid for inspecting the documents. However, expect to pay some substantial charges - possibly several pounds per page - if you require any photocopies).
Chris