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If your property hasn't been rewired in the last 25-30 years it's likely to need upgrading to at least bring it up to modern standards.
When planning major remodelling work that constitutes a material alteration as defined by the Building Regulations, you will need to rewire part, if not all, of the property including upgrading the consumer unit (fuse box).
When extending your home or converting part of it will constitute new work and therefore all of the new wiring will have to conform to Part P: Electrical Safety, and all existing wiring will have to be improved to ensure that it is able to carry the additional loads safely, it is earthed to current requirements and that cross bonding is satisfactory.
The rest of the existing wiring does not have to be upgraded.
Lots more information on rewiring can be found at http://www.homebuilding.co.uk/advice/key-choices/heating/wiring-explained