Is a mortgage in principle a guarantee that you'll get the loan?

No. A mortgage in principle — also called an agreement in principle, decision in principle, or AIP/DIP — is an indication of what a lender might be willing to lend based on information you have supplied and, usually, a credit check. It is not an offer and not binding on the lender.

What it is actually for is credibility. Estate agents routinely refuse to put offers forward without one, because it shows you are a plausible buyer rather than someone speculating. It also tells you roughly what budget you are working with before you fall in love with something unaffordable.

Why it can still fall through at full application:

The information has not been verified. An AIP relies on figures you typed in. The full application requires payslips, bank statements, accounts if self-employed, and identity documents, and any discrepancy changes the outcome.

Affordability is assessed properly. Full underwriting examines your actual spending — childcare, existing credit, subscriptions — against the declared figures.

The property itself must qualify. This catches people out constantly. A lender may decline non-standard construction, a flat above commercial premises, a short lease, a property with cladding issues, or one deemed unmortgageable for defects. Your finances can be perfect and the loan still refused.

Valuation. If the lender's valuer down-values the property, the loan is calculated on their figure.

Circumstances change. A job change, a new credit commitment or a missed payment between AIP and application can undo it.

Practical notes: an AIP typically lasts 30 to 90 days. Most now use a soft credit search that does not affect your file, but confirm before applying — and avoid collecting several from different lenders, as multiple hard searches do harm.

The binding document is the formal mortgage offer, issued after underwriting and valuation.