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Codicil vs New Will: The Safest Way to Update Mirror Wills After Life Changes (UK)

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If you and your partner have mirror wills, big life changes can quickly make them outdated — and the next question is usually:

“Do we need brand-new wills, or can we just add a codicil?”

This matters more than most people realise. Many inheritance disputes aren’t caused by having no will — they happen because a will was out of date, unclear, or accidentally contradictory after later changes.


Before you decide how to update your wills, read our main guide here:
Updating Mirror Wills After Major Life Changes: What Couples Need to Know
https://townandcountrylaw.legal/updating-mirror-wills-after-major-life-changes-what-couples-need-to-know/

Quick answer: codicil vs new will

  • Use a codicil if the change is small, clear, and you are not changing the structure of your will.
  • Write a new will if you are changing beneficiaries, adding children/stepchildren, changing property plans, separating/divorcing, or making multiple updates.

Rule of thumb:

  • ✅ One small change → a codicil may be suitable
  • ⚠️ Structural changes → a new will is usually safer
  • 🚩 Two or more codicils → a rewrite is often the clearest option
UK couple reviewing mirror wills and choosing between a codicil or a new will


Table Of Contents
  1. Quick answer: codicil vs new will
  2. What is a codicil (and why it can cause problems)?
  3. Codicil vs New Will (comparison table)
  4. When a codicil can be the right choice (simple changes)
  5. When you should usually write new mirror wills (higher-risk life changes)
  6. The biggest trap for couples: updating only one mirror will
  7. How to update mirror wills properly (step-by-step checklist)
  8. Common mistakes (and how to avoid them)
  9. FAQs
  10. Need help deciding whether a codicil is enough?

What is a codicil (and why it can cause problems)?

A codicil is a legal document that amends an existing will. It must be signed and witnessed correctly — you can’t just write changes on the will, cross things out, or staple notes to it.

Official guidance:
https://www.gov.uk/make-will/updating-your-will

Codicils can cause issues if they:

  • contradict clauses in the original will
  • refer to gifts/people that have changed since
  • are stored separately and can’t be found later
  • create uncertainty over which document is “latest”

Codicil vs New Will (comparison table)

Situation → Best option

  • Change an executor (simple) → Codicil often suitable (or new will also fine)
  • Change one small gift → Codicil often suitable (or new will also fine)
  • Add children / change guardians → New will usually best
  • Separation / divorce → New will usually best
  • Blended family / stepchildren → New will usually best
  • Property changes / new home → Often a new will (depends on complexity)
  • Multiple updates over time → New will is usually the cleanest solution

When a codicil can be the right choice (simple changes)

A codicil can work well when the structure of your will stays the same and you’re changing one specific point, for example:

  • replacing an executor (or adding a substitute)
  • changing a single cash gift amount
  • updating a beneficiary’s name (e.g., after marriage)
  • correcting a minor error

Important for couples: mirror wills are two separate documents. If you want them to remain “mirrored”, you’ll typically need two codicils (one per will) — and this is how couples sometimes drift out of sync.


When you should usually write new mirror wills (higher-risk life changes)

1) Marriage or civil partnership

In England & Wales, marriage can revoke an existing will unless it was made “in contemplation of” that marriage.

Wills Act 1837 (Section 18):
https://www.legislation.gov.uk/ukpga/Will4and1Vict/7/26/section/18

2) Separation or divorce

Relationship breakdown is a major trigger to review wills. Leaving everything to a partner may no longer reflect your wishes — and divorce can affect how certain gifts are treated.

Wills Act 1837 (Section 18A):
https://www.legislation.gov.uk/ukpga/Will4and1Vict/7/26/section/18A

3) Children, stepchildren, or blended families

This is where mirror wills most commonly become outdated. Guardian choices, fairness between children, and protecting a share for your own children can require clearer drafting than a simple amendment.

Helpful reading:

4) Buying/selling property or major asset changes

If you’ve moved home, bought a second property, or your finances have changed significantly, a rewrite often reduces risk and keeps your intentions clear.

5) Multiple changes over time

If you’re considering a second or third codicil, a new will is usually cleaner and easier for executors to administer.


The biggest trap for couples: updating only one mirror will

Because mirror wills are separate legal documents, it’s possible for one partner to update theirs while the other doesn’t. That can lead to:

  • different executors (causing delays)
  • inconsistent gifts (causing conflict)
  • contradictory “what if” outcomes
  • a higher chance of challenge (“why did one will change?”)

If the life change affects you both, it’s usually best to review and update both wills together.

How to update mirror wills properly (step-by-step checklist)

  1. Write down what changed (family, assets, relationships, responsibilities).
  2. Choose the method: one small tweak → codicil; structural change → new wills.
  3. Review executors (and replacements).
  4. Review guardians (children under 18).
  5. Check beneficiaries + backups (“what if they die before you?”).
  6. Think about protection planning (especially in blended families).
  7. Sign correctly with proper witnesses (don’t DIY edits on the original will).
  8. Store the latest signed versions safely and tell executors where they are.

Related mirror wills hub:
https://townandcountrylaw.legal/tag/mirror-wills/


Common mistakes (and how to avoid them)

  • Writing on the will and initialling it: can invalidate or cause confusion — use a codicil or rewrite.
  • Making a codicil, then losing it: store it with the original will (or professionally store it).
  • Only updating one mirror will: causes mismatch and increases dispute risk.
  • Using a template that doesn’t fit your family: risky for blended families and property planning.

FAQs

Can you change mirror wills?
Yes. Mirror wills are separate documents, so each person can change their own will at any time. Couples often update both together to keep them aligned.

Is a codicil legally valid in the UK?
Yes — if it’s properly drafted, signed, and witnessed. GOV.UK guidance:
https://www.gov.uk/make-will/updating-your-will

What’s safer: a codicil or a new will?
A new will is usually safer when you’re changing beneficiaries, family structure, property plans, or making multiple updates.

Do we need to update our wills after marriage?
Often yes. Marriage can revoke a will unless it was made in contemplation of that marriage (England & Wales):
https://www.legislation.gov.uk/ukpga/Will4and1Vict/7/26/section/18

Need help deciding whether a codicil is enough?

If you and your partner have had a major life change and you’re not sure whether you need a codicil or brand-new mirror wills, we can review your current documents and recommend the safest approach for your family and assets.

Will Writing Services:
https://townandcountrylaw.legal/will-writing-services/

Mirror Wills articles hub:
https://townandcountrylaw.legal/tag/mirror-wills/


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