If a property has not been rewired within the last 15 years or so, there is a possibility that much of the existing wiring will require upgrading to bring it in line with current standards. If you are considering major refurbishments that constitute a material alteration as defined in Part 2 of The Building Regulations 2010, there is a strong likelihood of needing a replacement consumer unit (fuse box) as well as substantial, if not full, rewiring.
Extensions and conversions comprise new work, and must therefore meet the Part P: Electrical Safety guidelines in full. In addition, all existing wiring needs to be brought up to standard to ensure that the additional load created by the new work can be handled safely and that cross bonding and earthing requirements are met.
We can generally tell whether a house or flat has been recently rewired by inspecting the consumer unit, electricity meter and any exposed areas. If you are looking at buying property, it is often a good idea to have these inspections made to avoid unexpected surprises. A house with an outdated fuse box, for example, or with a mixture of sockets and switches or surface-mounted wiring, may well require a complete rewiring.