How to Calculate the Right Sum Insured (Avoiding Underinsurance)

Choosing the right sum insured is one of the most important decisions when buying insurance—whether it’s for your home, health, or business. Set it too low, and you risk underinsurance, which can leave you paying large out-of-pocket costs when you make a claim.

This guide explains how to calculate it correctly—and how to avoid the costly mistakes many property owners make.

What Is “Sum Insured”?

The sum insured is the maximum amount your insurer will pay in the event of a claim. If this figure is inaccurate, your entire policy can be compromised.

For a deeper understanding of how insurers assess claims when values are incorrect, see this guide on the average clause in insurance.

What Is Underinsurance?

Underinsurance occurs when your sum insured is lower than the true replacement value of your property or assets.

Many policies apply an average clause, meaning your claim payout may be reduced proportionally if you are underinsured.

👉 This is one of the biggest financial risks property owners face—and often goes unnoticed until a claim is made.

Step-by-Step: How to Calculate the Right Sum Insured

1. Understand What You Need to Cover

Different policies require different approaches:

• Buildings insurance → rebuild cost
• Contents insurance → replacement value
• Landlord insurance → structure + liabilities
• Commercial insurance → assets, stock, and risks

If you’re a landlord, staying compliant with property standards (such as damp and mould regulations) is also essential when assessing risk and cover—see landlord compliance for damp and mould.

2. Calculate Rebuild Cost (Not Market Value)

A common mistake is confusing:

• ❌ Market value (includes land and location)
• ✅ Rebuild cost (actual cost to reconstruct the property)

Rebuild cost should include:

• Materials and labour
• Demolition and site clearance
• Professional fees (architects, surveyors)

3. Make a Detailed Inventory

For contents or business cover:

• List all major and minor items
• Include furniture, electronics, fixtures, and fittings
• Don’t forget cumulative value of smaller items

4. Factor in Inflation

Costs increase over time, especially in:

• Construction
• Labour
• Materials

Many policies include index linking, which helps adjust your sum insured automatically.

5. Include Hidden Costs

Often overlooked:

• Temporary accommodation
• Debris removal
• Specialist materials
• Delivery and installation

6. Consider Worst-Case Scenarios

Insurance is designed for total loss scenarios, not partial ones.

Ask yourself:

• What if the entire building had to be rebuilt?
• Could I afford the shortfall if underinsured?

7. Review Policy Limits and Sub-limits

Check for caps on:

• Jewellery
• Electronics
• High-value items

You may need to declare specific items separately.

8. Get Professional Advice

Working with a broker can help ensure:

• Accurate valuations
• Correct policy structure
• Competitive premiums

Many property owners are surprised by how much they can save while improving cover—especially when reviewing existing arrangements.

9. Reassess Regularly

Update your sum insured:

• Annually
• After renovations
• After major purchases

Changes in legislation and property management (such as recent Right to Manage updates) can also impact insurance responsibilities—see Right to Manage updates 2025.

Example: The Cost of Underinsurance

• Actual rebuild cost: £200,000
• Sum insured: £100,000 (50%)
• Claim: £40,000

With an average clause applied, you may only receive £20,000, not the full amount.

Common Mistakes to Avoid

• Using market value instead of rebuild cost
• Not reviewing cover regularly
• Ignoring inflation
• Underestimating contents
• Failing to understand policy conditions

Final Thought

Getting the sum insured right is about accuracy, not guesswork. Underinsurance can significantly reduce claim payouts and leave you exposed financially.

If you’re unsure whether your current cover is adequate, it’s always worth reviewing your policy or seeking expert advice. You can also request a quote or review your cover here to ensure you’re fully protected.