Vehicles of Historical Interest (VHI): Substantial Change Guidance
Most vehicles manufactured or first registered over 40 years ago will, as of 20 May 2018, be exempt from periodic testing unless they have been substantially changed1.
Large goods vehicles (i.e. goods vehicles with a maximum laden weight of more than 3.5 tonnes) and buses (i.e. vehicles with 8 or more seats) that are used commercially will not be exempted from periodic testing at 40 years.
A vehicle that has been substantially changed within the previous 30 years will have to be submitted for annual MoT testing.  Whether a substantially changed vehicle requires re-registration is a separate process.
Keepers of VHIs exempt from periodic testing continue to be responsible for their vehicle’s roadworthiness.  Keepers of vehicles over 40 years old can voluntarily submit vehicles for testing.   
Keepers of VHIs claiming an exemption from the MoT test should make a declaration   when renewing their vehicle tax.  The responsibility to ensure the declared vehicle is a VHI and meets the criteria, rests with the vehicle keeper as part of their due diligence.   If a vehicle keeper is not sure of the status of a vehicle, they can consult a marque or historic vehicles expert, a list of whom will be available on the website of the Federation of British Historic Vehicle Clubs.  
If a vehicle keeper cannot determine that the vehicle has not been substantially changed, they should not claim an exemption from the MoT test.                                                                 The criteria for substantial change
 A vehicle will be considered substantially changed if the technical characteristics of the main components have changed in the previous 30 years, unless the changes fall into specific categories.  These main components for vehicles, other than motorcycles, are:
Chassis (replacements of the same pattern as the original are not considered a substantial change) or Monocoque bodyshell including any sub-frames (replacements of the same pattern as the original are not considered a substantial change);
Axles and running gear – alteration of the type and or method of suspension or steering constitutes a substantial change;
Engine – alternative cubic capacities of the same basic engine and alternative original equipment engines are not considered a substantial change.   If the number of cylinders in an engine
is different from the original, is likely to be, but not necessarily, the case that the current engine is not alternative original equipment.

The following are considered acceptable (not substantial) changes if they fall into these specific categories: 
 
changes that are made to preserve a vehicle, which in all cases must be when original type parts are no longer reasonably available;
 
changes of a type, that  can be demonstrated to have been made when vehicles of the type were in production or in general use (within ten years of the end of production);
 
in respect of axles and running gear changes made to improve efficiency, safety or environmental performance;
 
in respect of vehicles that have been commercial vehicles, changes which can be demonstrated were being made when they were used commercially.

In addition if a vehicle (including a motorcycle):
 
• has been issued with a registration number with a ‘Q’ prefix; or • is a kit car assembled from components from different makes and model of vehicle; or • is a reconstructed classic vehicle as defined by DVLA guidance; or   • is a kit conversion, where a kit of new parts is added to an existing vehicle, or old parts are added to a kit of a manufactured body, chassis or monocoque bodyshell changing the general appearance of the vehicle;
 
it will be considered to have been substantially changed and will not be exempt from MOT testing.   
 
However if any of the four above types of vehicle is taxed as an “historic vehicle” and has not been modified during the previous 30 years, it can be considered as a VHI.
 This guidance is only intended to determine the testing position of a substantially changed vehicle, not its registration.