Agreement in Principle (AIP) Explained: The 2-Day Mortgage Step That Wins UK Property Offers

Key Takeaways

• Provisional, not binding: An AIP is a lender's indication based on soft-check data, not a formal mortgage offer. Estate agents treat it as your credibility signal.
• Two days to get, 90 days to use: Most straightforward AIPs issue within 48 hours. Validity is 60 to 90 days depending on lender.
• BTL versions have specific tests: Rental cover ratio at 125 to 145 percent depending on tax band. Stress rate at 5.5 percent or product pay rate plus 2 percent.
• Off-plan requires two AIPs: The one at reservation lapses long before completion. Investors serious about off-plan get a second AIP six to eight weeks before completion.

What is Agreement in Principle (AIP)?

An Agreement in Principle (AIP), also called a Decision in Principle (DIP) or Mortgage in Principle (MIP), is a written indication from a mortgage lender that they would in principle be willing to lend you a certain amount. It is not a legally binding promise. It is a preliminary check based on your income, existing commitments and a soft credit search.

The document itself is short: usually a one or two-page letter confirming the lender name, the approximate amount you can borrow, and an expiry date. Estate agents and vendors often ask to see it before accepting your offer, which is where its practical power sits. Two identical offers land on a developer's desk; the one with an AIP attached tends to be taken more seriously. Unfamiliar terminology? Our investor glossary covers the fundamentals.

Quick FAQ:

Q: Is an Agreement in Principle the same as a mortgage offer?

A: No. An AIP is a preliminary indication based on limited checks. A formal mortgage offer follows a full application, hard credit check, income verification, property valuation and underwriting review. Only the formal offer is legally binding on the lender.

AIP vs formal mortgage offer

The two documents are often confused because they can both be labelled as "mortgage approval" in casual conversation. They are not the same thing.

How buy-to-let AIPs differ from residential

The core mechanics are identical, but the lending tests are not. A BTL AIP looks at your personal position AND the future rental income of the property you plan to buy. Two specific tests apply:

Interest Cover Ratio (ICR): Rental income must exceed the stressed mortgage interest by a set multiple. For basic-rate taxpayers and limited company borrowers, this is typically 125 percent. For higher-rate and additional-rate taxpayers borrowing in personal name, it is 145 percent.

Stress rate: Lenders apply a stress rate that is the higher of 5.5 percent or the product pay rate plus 2 percent. For two-year fixes, this can push the stress test rate above 7 percent. Five-year fixes usually stress-test lower because the longer fix period reduces short-term refinancing risk.

Practically, BTL AIP amounts are often lower than residential AIP amounts on the same income, and the rental yield of the property matters as much as your salary.

Quick FAQ:

Q: What rental cover ratio does a BTL AIP need?

A: 125 percent for basic-rate taxpayers or limited-company borrowers, 145 percent for higher-rate or additional-rate taxpayers borrowing in personal name.

Related Reading

Three pieces build the fuller picture on this story:

• UK Mortgageable Lease Length: The Threshold Every Flat Investor Needs to Know — Your AIP is only useful if the property qualifies too. This covers the property-side check for leasehold flats.
• First-Time Buy-to-Let: A Practical Guide for UK Investors — Where the AIP sits in the wider first-purchase process, end-to-end.
• Bank of England Holds at 3.75%: Why Buy-to-Let Investors Should Read Between the Lines — How base rate movement resets AIP stress tests, sometimes uplifting borrowing capacity mid-search.

The AIP process step by step

For a straightforward case, the process runs in five stages and is usually complete within 48 hours.

1. Speak to a broker or lender: A broker will compare rates across the market and match you to a lender likely to approve. Going direct is faster but limits you to that lender's criteria.

2. Submit basic information: Income, employment status, existing debts, savings, ID and address history. For BTL, also expected rental income and property value.

3. Soft credit search: The lender pulls a soft credit file visible only to you. Does not affect your credit score.

4. AIP issued: Usually within 24-48 hours for uncomplicated cases. Document confirms the borrowable amount and expiry date.

5. Use it to make offers: Attach the AIP to any offer letter. Estate agents may photocopy it. Off-plan developers typically require sight of one before accepting a reservation.

Quick FAQ:

Q: Should I use a broker or go directly to my bank for an AIP?

A: A broker compares dozens of lenders and matches you to the criteria most likely to approve, which matters more for buy-to-let than residential. Going directly to your bank is faster if you already know their criteria fit, but limits your rate options. For a first BTL, a broker is usually worth the two-day wait.

What can invalidate your AIP

An AIP is not permanent. Any of the following will typically require a re-issue, and in some cases will reduce or refuse the amount:

Change in income or employment: A job change, especially into self-employment or contract work, resets the lender's risk view.

New debt taken on: A car loan, credit card balance increase, or "buy now pay later" arrangement all count.

Missed payments: Any missed direct debit, minimum credit-card payment or utility default in the 30 days after issue.

Base rate movement: If the Bank of England moves rates materially, some lenders re-price and existing AIPs are recalculated.

Property-side issues: Your AIP is only useful if the property itself qualifies. For leasehold flats, mortgageable lease length is a common failure point, see the lease-length guide linked in Related Reading above.

Quick FAQ:

Q: How long is a mortgage AIP valid for in the UK?

A: Between 60 and 90 days, depending on lender. Barclays and NatWest typically issue for 90 days. HSBC for 30 days. If you need longer, most lenders will re-issue with a fresh soft check.

Off-plan property and AIP timing

Off-plan is where AIP timing becomes a real puzzle. You reserve a property today. Construction completes in 12 to 30 months. Your AIP is valid for at most 90 days.

The workaround most brokers use is a two-stage approach. First, you obtain an AIP at reservation to confirm the developer's affordability check. That AIP is used solely for the reservation, then allowed to lapse. Second, six to eight weeks before scheduled completion, you obtain a fresh AIP for the same amount (or higher if rates have moved favourably), which is then converted into a full mortgage application in the final four to six weeks.

This matters because mortgage rates in 30 months will not be the same as today's. A shrewd off-plan investor knows the reservation-stage AIP is a formality; the real AIP is the one obtained near completion.

Explore: Velocity, Trafford

Velocity is a live Rothmore Manchester development in the £7bn Trafford regeneration zone. Off-plan, from £209,900, yields up to 6.3%, completing Q3 2028. Chosen for this piece because Velocity is where the two-stage AIP dance actually plays out, reservation AIP now, refresh AIP six to eight weeks before Q3 2028 completion.

Summary for investors

An Agreement in Principle costs nothing, takes two days, and lasts 90 days. It is the lowest-cost move any UK property buyer can make to improve their negotiating position, and in the off-plan market it is often a hard requirement at reservation. The mistake most first-time investors make is not skipping it entirely but treating it as a formal offer. Understand what it is and is not, time the second AIP around completion, and you close cleaner.

Disclaimer:

Rothmore Property is not a mortgage broker, financial adviser or tax adviser. This article is for general information only and does not constitute mortgage, investment, tax or legal advice. Mortgage products and lender criteria change frequently. Property values can fall as well as rise. Investors should take independent professional advice from an FCA-regulated mortgage broker and, where relevant, an accountant before committing to any borrowing or purchase.