Mortgage in Principle vs Mortgage Offer: What's the Difference?

When you're buying a property in the UK, you'll encounter two very different documents from your lender: a mortgage in principle (also called an Agreement in Principle or Decision in Principle) and a formal mortgage offer. Understanding the difference between a mortgage in principle and a mortgage offer could save you from a collapsed chain, a damaged credit score, or a nasty shock when your application is declined. This guide explains what each document is, when you need it, and the five things most likely to kill a mortgage offer after it's been issued.

For the full house buying process — from getting an AIP to collecting the keys — see our step-by-step guide to buying a house. For a complete overview of mortgage types and which best suits your circumstances, see our mortgage types explained guide.

What Is a Mortgage in Principle?

A mortgage in principle (MIP) — also known as an Agreement in Principle (AIP) or Decision in Principle (DIP) — is a lender's conditional indication that it would be willing to lend you a specified amount, based on limited information. It is not a guarantee.

To issue an AIP, a lender typically asks about:

Your income (employed, self-employed, or both)
Your monthly outgoings and existing debts
The size of your deposit
Your address history
Your credit status (via a soft or hard credit check — see below)

The AIP is used primarily to demonstrate to estate agents and sellers that you are a serious buyer who has at least provisionally been approved for the funds you need. Some estate agents will not show you properties or put forward your offer without one.

Soft vs Hard Credit Search at AIP Stage

This is where many buyers unknowingly create problems. When a lender runs an AIP, it will conduct either a soft credit search or a hard credit search:

Soft credit search: Leaves no visible trace on your credit file to other lenders. You can apply to multiple lenders for AIPs without affecting your credit score.
Hard credit search: Leaves a visible footprint on your credit file for 12 months. Multiple hard searches in a short period can signal financial stress to lenders and reduce your chances of a successful mortgage application.

Always ask your lender or broker whether the AIP will involve a hard or soft search. Most AIPs use soft searches, but some lenders still conduct hard searches at this stage. If you are shopping around, use a whole-of-market broker who can identify the right lender before any searches are run.

How Long Does an AIP Last?

Most AIPs are valid for 60 to 90 days. If your property search takes longer than that, you will need to refresh the AIP, which may involve another credit check.

What Is a Formal Mortgage Offer?

A formal mortgage offer is a legally binding commitment from a lender to lend you a specific amount on specific terms, against a specific property. It can only be issued once the lender has:

Fully verified your income and employment (payslips, SA302s for self-employed, bank statements)
Completed a credit check (always a hard search at this stage)
Received a satisfactory valuation or survey of the property
Carried out affordability stress-testing against their current criteria

A mortgage offer is typically 30–60 pages and specifies the loan amount, interest rate, type (fixed/tracker/variable), monthly payment, term, and any conditions that must be met before drawdown.

How Long Does a Mortgage Offer Last?

Most mortgage offers are valid for 3 to 6 months from the date of issue. New-build properties are often given extended offers of up to 12 months, because completion dates can be uncertain. If your offer expires before completion, you will need to reapply — potentially at a different rate.

AIP vs Mortgage Offer: Comparison Table

Feature	Agreement in Principle	Formal Mortgage Offer
Legally binding on the lender	No	Yes
Credit check type	Soft (usually) or hard	Always hard
Income documentation required	Self-declared	Fully verified
Property-specific	No	Yes
Valuation/survey required	No	Yes
Time to receive	Minutes to 24 hours	2–6 weeks after application
Validity	60–90 days	3–6 months (up to 12 months for new build)
Can be used to make an offer on a property	Yes	N/A (post-offer stage)

What Can Cause a Mortgage Offer to Differ From the AIP?

Many buyers are caught out when their formal mortgage offer comes back for less than the AIP suggested — or is declined altogether. Common causes include:

1. Survey Down-Valuation

If the lender's valuer assesses the property as worth less than the agreed purchase price, the lender will offer based on the lower figure. A £300,000 property valued at £280,000 means the lender will only lend against £280,000 — leaving you to make up the shortfall or renegotiate the purchase price.

2. Changed Income

If your income has changed between AIP and full application — redundancy, a change of job, reduced hours, the end of a fixed-term contract — the lender's calculations will produce a different result. Lenders also count furlough, probationary periods and zero-hours contracts differently.

3. New Debts or Deteriorated Credit

A new credit card, car finance, or a missed payment taken out or occurring after the AIP can change your affordability assessment significantly.

4. Property Type Issues

Lenders have specific restrictions on certain property types — non-standard construction, flats above commercial premises, properties near former industrial land, ex-local authority flats above a certain height, short leases. If the property triggers a restriction, the lender may decline or offer on different terms.

5. Self-Employment Income Complexity

Self-employed applicants whose SA302s show declining income, director's dividends varying significantly year-on-year, or retained profits not taken as salary may find the lender's formal assessment differs markedly from the self-declared AIP figures.

5 Things That Kill a Mortgage Offer After It's Issued

Taking on new credit. Any new credit agreement — even a 0% purchase card or a new phone contract — can trip automated monitoring systems. Avoid all new credit between offer and completion.
Changing jobs. Lenders often re-verify employment immediately before completion. A new job, especially in a probationary period, can cause the offer to be withdrawn.
The property failing a subsequent inspection. If the property is damaged between exchange and completion (fire, flood, structural movement), the lender may reassess.
Material changes to the property. Discovering the property has planning breaches, building regulations non-compliance, or undisclosed alterations can cause a lender to revisit the offer.
Fraud or misrepresentation. If the lender discovers that income figures, employment status or property use were misrepresented during the application, the offer will be withdrawn and may be referred to the National Hunter fraud database.

What to Do If Your Mortgage Offer Expires

If your mortgage offer expires before completion — typically because of a delayed chain, a slow conveyancing process, or a new-build overrunning — you have three options:

Request an extension from your current lender. Many lenders will extend by one to three months, sometimes free of charge, sometimes for a modest administration fee of £100–£300. There is no guarantee, and the lender may require updated payslips or a refreshed valuation.
Reapply with the same lender. If an extension is refused, you can submit a fresh application. You will need to go through the full underwriting process again, and the rate may have changed.
Switch to a different lender. If your circumstances have changed or better rates are now available, a fresh application elsewhere may be the right move. This will involve another hard credit search.

Practical Takeaways

Always ask whether an AIP will involve a soft or hard credit check. Soft searches leave no trace; hard searches leave a 12-month footprint.
An AIP is not a mortgage — it is a preliminary indication only. Never rely on it as confirmation of funding.
A formal mortgage offer is legally binding on the lender but can still be withdrawn if circumstances materially change.
Avoid taking out any new credit, changing jobs, or making large financial changes between receiving your mortgage offer and completing.
If your offer is at risk of expiring, contact your lender or broker immediately — extensions are much easier to arrange in advance than at the last minute.
Renovate Me's renovation roadmap includes a funding stage that prompts you to get an AIP before viewing properties and flags the optimal timing for your full application.

Mortgage in Principle vs Mortgage Offer: What's the Difference?