What a mortgage in principle actually commits you to
The short answer
It commits nobody to anything. A mortgage in principle is a lender saying that, on the information you have given it, it would probably lend you roughly this much. It is not an offer, and the lender can walk away from it at any point.
Its real value is credibility with estate agents. It says you are a proceedable buyer rather than someone who has not yet checked. In a competitive market that is often the difference between an offer being taken seriously and being ignored.
Getting one is free, quick and usually harmless to your credit file, because most major lenders now use a soft search. That is worth confirming before you apply rather than after.
The names all mean the same thing
Agreement in principle, decision in principle, mortgage in principle, AIP, DIP, MIP. Lenders use different labels for an identical product and the choice of wording carries no meaning. If someone tells you their lender does a decision in principle rather than an agreement in principle, they are describing the same thing.
You may also hear the American term pre-approval used, usually by property websites. It is not a UK term and it overstates what you have. Nothing has been approved.
What the lender has actually done
Very little, which is the point. It takes minutes rather than weeks because almost nothing is checked.
The lender takes the income, outgoings and deposit you have typed in, runs a light credit check, applies its affordability model and returns a figure. It has not seen a payslip, a bank statement or a tax calculation. It does not know which property you are buying, so it has not valued anything or considered whether the property is one it would lend on at all. It has not underwritten your case in any meaningful sense.
So the figure is a projection of your own numbers through the lender's model. If your numbers were optimistic, so is the figure.
Before you apply Mortgage affordability calculatorSanity-check your expectations first, so the figure the lender returns is a confirmation rather than a surprise.
Will it damage your credit score?
Almost certainly not, but it is worth understanding why rather than taking it on trust.
A soft search is recorded on your credit file but only you can see it. Other lenders cannot, and it has no effect on your score. A hard search is visible to every lender who looks for the next twelve months, and several in quick succession can suggest you are being turned down repeatedly.
The major high street lenders have moved to soft searches at this stage over the past few years, and several now say so explicitly on their own pages. Some smaller building societies and specialist lenders still run a hard search, and applications made in branch are sometimes handled differently from the same lender's online route.
The full application later is always a hard search. That one is unavoidable and expected, and a single hard search when you are actually buying a house is not something to worry about.
How long it lasts
Somewhere between 30 and 90 days, with most high street lenders sitting at 60 or 90. A small number are shorter, around a month, and one or two run longer.
We have deliberately not published a lender-by-lender table here. These periods change quietly, without announcement, and a table like that is wrong within months while still looking authoritative. Check the figure on your own lender's page, which is the only version guaranteed to be current.
Expiry is not a problem in itself. If yours runs out while you are still looking, most lenders will renew it, and if nothing has changed the renewal is usually quick. The thing to avoid is discovering it has expired on the day an agent asks to see it.
What quietly invalidates it
Because it is a snapshot of a moment, anything that changes the underlying picture can change the answer. The common triggers are worth knowing in advance, because several of them are things people do without realising there is a mortgage consequence.
- Taking on new credit. A car on finance, a credit card, a buy now pay later arrangement or an interest-free sofa all count. Because commitments are deducted before the income multiple is applied, a modest monthly payment can remove a surprising amount of borrowing capacity.
- Changing jobs. Even a pay rise can go against you if the new role is on probation or is a move into self-employment or contracting.
- Any drop in income, including going part time, unpaid leave, or maternity or paternity leave starting.
- A missed payment on anything at all, including a phone contract.
- Becoming a guarantor for someone else, which lenders treat as a commitment of yours.
The practical rule between agreement in principle and completion is simple: change nothing financially that you do not have to change.
How it differs from a mortgage offer
These are frequently confused, and the gap between them is where most disappointment happens.
| Agreement in principle | Formal mortgage offer | |
|---|---|---|
| Based on | What you typed in, plus a light credit check. No documents, no property. | Verified income and outgoings, full credit search, valuation of the specific property, legal checks. |
| Legal weight | None. Indicative only, and the lender can withdraw at any time. | A binding commitment to lend on that property at those terms, subject to its stated conditions. |
| Typical lifespan | Around 30 to 90 days. | Around three to six months, and extendable in most cases. |
| Time to get one | Minutes. | Weeks, sometimes considerably longer. |
| What it is for | Showing agents and sellers you are serious, and setting your own budget. | Actually buying the house. |
What an estate agent can and cannot require
This is where an agreement in principle stops being a mortgage question and becomes a consumer rights one, and it is worth reading even if you already have one.
There is no legal requirement to hold an agreement in principle before viewing a property or making an offer. Agents may nevertheless set their own policy, and in busy markets many will not book viewings without some evidence you can fund the purchase. That is a legitimate commercial filter, and an agreement in principle from any lender, a letter from a broker, or proof of cash all satisfy it.
What an agent cannot do is make any of it conditional on using their own services. Refusing to pass on your offer unless you see their in-house broker, delaying it, or treating you less favourably for declining, is conditional selling. It is a prohibited practice under the Estate Agents (Undesirable Practices) (No. 2) Order 1991, made under the Estate Agents Act 1979, and it applies across England, Wales, Scotland and Northern Ireland.
National Trading Standards guidance for estate agents is explicit that agents must not discriminate against buyers who decline their associated services, and must not attach extra conditions of their own before passing an offer on. The Property Ombudsman's code, which member agents are bound by, separately requires all offers to be passed to the seller promptly.
One practical note. Agents sometimes ask to see the agreement in principle document itself, which shows the amount you have been approved for. You are entitled to redact that figure, or to supply a broker's letter confirming you are funded instead. An agent who knows your ceiling is negotiating with information you did not need to hand over, and their duty is to the seller rather than to you.
Why an application can still be declined afterwards
An agreement in principle is not a promise, and hearing yes then no later is more common than most buyers expect. The usual reasons fall into a few groups.
Affordability tightens. The full assessment uses verified figures and stricter stress testing than the initial model. Outgoings visible on bank statements are often higher than the ones people estimate from memory.
The documents do not match. Income declared as a round number, bonus treated as guaranteed when it is not, or an address history that does not line up. Inconsistencies raise questions even when entirely innocent.
The full credit search finds something. A soft search at the earlier stage may not have surfaced everything, and information held by a different credit reference agency from the one the lender checked first can appear late.
The property fails, not you. A down valuation, a short lease, non-standard construction, cladding, or a flat above commercial premises can all end an application that was fine on affordability. This has nothing to do with your finances and is the most common reason a perfectly good buyer is declined.
The lender changes its mind about lending generally. Products get withdrawn and repriced, and risk appetite tightens. None of that is about you.
Related guide How lenders decide what you can borrowWhy two lenders can look at identical paperwork and arrive at figures tens of thousands of pounds apart.
The sensible order to do things in
Do not treat the figure as a target
An agreement in principle produces a number, and numbers presented as approvals have a way of becoming budgets. It is a ceiling calculated on today's circumstances by an organisation that does not know about the career break you are considering, the boiler that is nearly finished, or how the payment would feel if rates were higher at the end of your fixed period.
Borrowing to a payment you have chosen, rather than to the limit a model produced, is a decision very few people regret.
The number that matters Monthly budget calculatorWhat a given mortgage payment would actually leave you with each month, once everything else is accounted for.