A mortgage agreement in principle (AIP) — sometimes called a decision in principle or mortgage in principle — is a statement from a lender saying that, based on the information you've provided, they'd be willing in principle to lend you a certain amount. It's not a mortgage offer, but it's often the first real hurdle in buying a home, and estate agents will frequently ask for one before letting you view a property, let alone put in an offer.

Why estate agents insist on one

From an agent's and seller's perspective, an AIP is evidence that a buyer isn't wasting everyone's time — that a lender has done a basic check and believes, provisionally, that you can afford the mortgage you'd need. In a competitive market, sellers will often favour an offer backed by an AIP over one without, since it reduces (though doesn't eliminate) the risk of the sale falling through later for financial reasons.

What the lender actually checks

To issue an AIP, a lender typically carries out a "soft" credit check and asks about:

  • Your income and employment status
  • Your outgoings and existing debts
  • The deposit you have available
  • Your credit history, at a high level

Because this is usually based on the information you provide rather than full documentary proof, the AIP is provisional — it isn't a guarantee the mortgage will actually be approved once a lender verifies everything with payslips, bank statements and a full credit search at the formal application stage.

Soft search vs hard search — why it matters for your credit file

Most lenders now use a "soft" credit search to generate an AIP, which is visible only to you and doesn't affect your credit score or leave a mark other lenders can see. However, some lenders still perform a "hard" search at this stage, which is recorded on your credit file and visible to other lenders — applying for several AIPs in a short space of time with lenders that use hard searches can look like multiple credit applications and potentially harm your score. It's worth checking which type a lender uses before applying, especially if you're comparing several.

How long an AIP lasts

Most agreements in principle are valid for around 30 to 90 days, depending on the lender, after which you'd need to renew it if you haven't yet found a property or completed a full application. It's generally sensible to get an AIP close to when you actually start seriously viewing properties, rather than months in advance, so it doesn't expire at an inconvenient moment.

What it doesn't guarantee

Common reasons a full application later differs from the AIP include: the lender's full credit check reveals something the soft search didn't, your circumstances change (new debt, job change), or the property itself fails to meet the lender's valuation or lending criteria (for example, being a non-standard construction or having a short lease).

What to do next

Use a mortgage broker or comparison tool to get an AIP from a lender likely to suit your circumstances before you start viewing seriously. Keep your financial situation stable in the run-up to a full mortgage application — avoid new credit commitments, large unexplained transactions, or changing jobs if you can help it, since these are exactly what a full underwriting check will scrutinise.