Classic car modifications and insurance: what you must declare

For many classic car owners, modification is part of the joy of ownership. Whether it’s improving reliability for touring, enhancing performance, or restoring a vehicle to period-correct specification, modifications are often carefully considered and sympathetically executed. 

However, when it comes to classic car insurance in the UK, one rule is absolute: if you’ve modified your vehicle, you must declare it. 

Failing to disclose modifications could invalidate your cover, leaving you exposed in the event of a claim. In this guide, we explain what counts as a modification, what you need to tell your insurer, and how specialist classic insurers such as Footman James approach modified vehicles. 

In insurance terms, a modification is any change from the manufacturer’s original specification. This includes alterations made after the vehicle left the factory - even if they were completed decades ago. 

Modifications generally fall into the following categories: 

1. Performance modifications 

These are changes that affect the vehicle’s speed, handling, or power output. Examples include: 

• Engine upgrades or swaps 

• Modified carburettors or fuel injection systems 

• Turbochargers or superchargers 

• Uprated camshafts 

• Performance exhaust systems 

• Suspension upgrades 

• Brake conversions 

Even if the modification improves safety or drivability, it must be declared. 

2. Cosmetic modifications 

Changes to the vehicle’s appearance can also affect insurance: 

• Alloy wheels not originally fitted 

• Body kits or spoilers 

• Custom paintwork or wraps 

• Non-standard interiors 

• Steering wheel replacements 

Even period-style upgrades may count if they were not part of the original specification. 

3. Structural or chassis changes 

Alterations to the vehicle’s structure or layout are particularly important to declare: 

• Chassis modifications 

• Convertible conversions 

• Roll cages 

• Suspension geometry changes 

These can significantly affect risk assessment and vehicle value. 

4. Modernisation and “restomod” upgrades 

Increasingly popular in the classic car community, restomods blend classic aesthetics with modern mechanicals. Examples include: 

• Electronic ignition conversions 

• Disc brake upgrades 

• Power steering additions 

• Electric powertrain conversions 

• Modern gearbox installations 

Even upgrades aimed at improving reliability or usability must be disclosed. 

Yes. It doesn’t matter whether the modification was carried out last month or in 1975 - if it differs from factory specification, it should be declared. 

For example, if your 1967 MGB was fitted with aftermarket wheels in the 1980s and has worn them ever since, they are still technically a modification. 

Similarly, dealer-fitted accessories may need clarification if they were not part of the standard production build. 

If you’re unsure, it’s always safer to inform your insurer. 

When arranging classic car insurance, insurers assess risk based on accurate information about the vehicle. Undeclared modifications can create several issues: 

• Policy invalidation – Non-disclosure may result in a refused claim. 

• Reduced payout – If the vehicle’s true value isn’t declared, you may not receive an adequate settlement. 

• Policy cancellation – In serious cases, cover may be withdrawn. 

Classic cars are often insured on an agreed value basis, meaning you and the insurer confirm the vehicle’s value upfront. Modifications can significantly influence that valuation - sometimes increasing it. 

Full transparency ensures you’re properly protected. 

There is a common misconception that declaring modifications automatically increases premiums. With specialist insurers such as Footman James, that isn’t necessarily the case. 

Premium impact depends on factors including: 

• The type of modification 

• Whether it increases performance 

• The vehicle’s usage 

• Storage and security arrangements 

• Owner experience 

Some modifications - such as safety upgrades (e.g., improved brakes or electronic ignition) — may not materially increase risk and could even be viewed positively. 

Performance enhancements, however, may influence underwriting decisions. 

The key advantage of working with a specialist insurer is their understanding of classic car culture. They recognise that modifications are often sympathetic and carefully engineered rather than reckless alterations. 

Engine swaps are one of the most significant modifications from an insurance perspective. 

You must declare: 

• Engine size changes 

• Different engine types 

• Power output increases 

• Non-original manufacturer engines 

For example, fitting a Rover V8 into a classic Land Rover, or upgrading a classic Ford with a modern Zetec engine, represents a material change. 

Failure to declare an engine swap could invalidate your policy entirely. 

Imported vehicles and rebuilt classics deserve particular attention. 

If your vehicle: 

• Has been rebuilt using parts from multiple vehicles 

• Has a replacement chassis 

• Has undergone major restoration beyond factory specification 

• This must be disclosed clearly. 

Vehicles registered as reconstructed classics or radically altered vehicles may require specialist underwriting consideration. 

The rise of electric classic conversions presents new insurance considerations. 

Converting a petrol or diesel classic to electric power fundamentally alters: 

• The powertrain 

• weight distribution 

• Mechanical layout 

• Vehicle value 

These conversions must always be declared and insured through a provider experienced in modified or specialist vehicles. 

Maintaining documentation helps protect both you and your insurer. Keep: 

• Receipts for parts and labour 

• Photographs of work completed 

• Engineering reports (where relevant) 

• Dyno results (if performance-related) 

These records support agreed value assessments and make the underwriting process smoother. 

You should inform your insurer: 

• At the time of taking out the policy 

• Before making new modifications 

• Immediately after any significant changes 

• Never wait until renewal to disclose recent upgrades. 

Modifying a classic car is part of the passion of ownership. Whether you’re preserving originality, improving reliability for touring, or building a bespoke restomod, your insurance policy should reflect your vehicle accurately. Declaring modifications isn’t about creating complications - it’s about ensuring your vehicle is fully protected.