Not automatically, and this catches out more landlords than the lease or the 90-night rule combined. A standard buy-to-let mortgage is priced and underwritten around a long-term tenancy, usually an assured shorthold tenancy, and most lenders' terms and conditions say so explicitly. Short-letting on a product like that without asking is very often a breach of the mortgage agreement, not a grey area you can reasonably assume your way around.
Why the product matters, not just the lender
A buy-to-let mortgage is not a generic loan against a rental property. It is priced on an assumption about how the property will be occupied and by whom, because that assumption is what the risk is based on. A long tenancy has a vetted tenant, a fixed rent, and a contract running for months or years. A short let has a different guest every few days, no reference checks in the usual sense, and income that moves with demand rather than sitting fixed.
Lenders treat that as a materially different risk, which is why the standard buy-to-let product usually does not cover it. It is not that lenders have not thought about short lets. It is that the product you have was built for something else, and the terms say so.
This is true whether the flat is in central London or anywhere else. It has nothing to do with the 90-night planning limit, which is a separate rule about land use. You can be entirely within the 90-night cap and still be in breach of your mortgage, because the two systems do not talk to each other and one does not excuse the other.
What lenders actually do about it
This varies a great deal, and it is worth being specific rather than treating "lenders" as one position.
Some standard buy-to-let lenders will give consent for short lets on request. This usually means writing to the lender, sometimes filling in a form, and sometimes paying a fee or accepting a change in terms. Some will say yes without much friction if the property is well-managed and insured properly. Some will say no outright.
Some lenders offer a specific consumer buy-to-let or holiday-let product built for short-term and seasonal letting from the start. These are underwritten differently, often expect the property to be professionally managed, and sometimes ask for evidence of demand or projected income before lending against it. If you know from the outset that a property will be run short-term, this is usually the more honest route than starting on a standard product and asking for an exception later.
Some lenders will not permit it under any product they currently offer. This is not rare, and it is worth finding out before you buy a property with short lets in mind, not after.
Whichever category your lender falls into, the answer is specific to that lender and that mortgage, not to buy-to-let mortgages generally. Two landlords with flats on the same street can have entirely different answers depending on who they borrowed from.
What usually happens if you never ask
Nobody phones you the week you list on Airbnb. Mortgage lenders do not monitor short-let listings as a matter of course, and plenty of landlords run short lets on a standard buy-to-let product for years without anything happening.
The risk is not a routine check. It is what the lender can rely on if something else goes wrong. If there is a claim, a dispute, a remortgage, or the lender finds out for any reason, unauthorised letting is a breach they are entitled to act on. In practice this can mean being asked to remortgage onto a different product immediately, being asked to stop short-letting, or in a serious case being in breach of the loan agreement itself. Insurance can compound this: if the mortgage terms required certain letting conditions and those were not met, an insurer may also have grounds to decline a claim, quite separately from whether the insurance itself was ever suited to a short let in the first place.
None of this is a fine or a fee we can quote, because it depends entirely on the lender, the specific breach, and what has actually happened. That is exactly why it is worth settling in advance rather than finding out the details after the fact.
What to actually do
Ring the lender, or write to them, and ask directly: does this mortgage permit letting the property on a short-term, nightly basis, and if not, is consent available. Keep the answer in writing. If the answer is no and the product cannot be changed, that is worth knowing before a single guest checks in, not after a year of income you would then have to unwind.
If you are buying with short lets already in mind, mention that to the broker before you apply, so you end up on a product built for it rather than one you later have to ask permission to use differently.
This sits alongside the lease, if the property is leasehold, and the insurance, both of which have their own view on short-letting entirely separate from what the mortgage says. All three have to clear before a property is genuinely free to list.
If you already know your mortgage permits short lets, or you want a second opinion on what a lender is likely to say, send us the postcode and we will tell you honestly what else is worth checking before you list.
This is general information about how buy-to-let mortgages typically treat short-letting, not financial advice. Terms vary significantly between lenders and between products from the same lender. Confirm your position directly with your lender or a mortgage broker before letting a mortgaged property short-term.