At worst, faulty electrics can be a serious fire hazard and can lead to injury or electrocution. This is why electrics were brought in to the building regulations under Part P. Existing wiring installations are not covered, but if you make significant alterations or new additions to the wiring in your home, the regulations come into force.
If you are thinking of buying a property more than 25 years old, it is important to check that the wiring is up to date before you buy. Ideally, you will get an idea of what work is required and an estimate of the likely cost so that you can take this into account when assessing the feasibility of the project and how much to offer.
Even if you already own the property you are renovating, it is important to know early on if any rewiring work will be necessary, as it can be very disruptive to the fabric and decor of the building, and so is best completed early on, before any re-plastering or redecorating work takes place.
When is Rewiring Necessary?
If a property has not already been rewired within the last 25-30 years, the chances are it will need upgrading at least in part in order to bring it up to current standards. The wiring may be potentially dangerous and may not be able to cope with the demands of modern living.
If you plan major re-modelling work that constitutes a material alteration as defined by the Building Regulations, it is likely that you will need to rewire part, if not all, of the property, including upgrading the consumer unit (fuse box).
If you are extending your home, or converting an attic or garage, this 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.
Where you are extending or re-modelling, the rest of the existing wiring does not have to be upgraded, except where upgrading is required by the energy efficiency requirements of the Building Regulations, i.e. central heating controls.
What if it has not Been Rewired?
Unless the wiring is the modern PVCu coated type, then a rewire is likely to be necessary. If you see any old rubber insulated cabling, fabric insulated cabling (used until the 1960s), or lead insulated cabling (used until 1955) then it needs replacing as the insulation can rot and/or break down, leading to short circuiting: a fire hazard and potential electrocution.
Even older PVCu cable may need replacing if it is not twin earthed cabling (with a second earth cable running within the outer sleeve), but this may only be evident if you are able to remove a switch or socket faceplate and look closely.
Checking for this conducting the viewing, and certainly is not advisable unless you are able to turn off the mains first.
If you are in any doubt, assume that a total rewire is required and budget accordingly. It may be that the system can be improved for less money by upgrading earthing and cross bonding.