Buy to Let vs Residential Mortgage

You cannot generally use a residential mortgage to buy a property you intend to rent out, and you cannot simply start letting a property you bought on a residential mortgage without telling your lender. Both are normally a breach of your mortgage terms, because a residential mortgage is priced and underwritten on the basis that you will live in the property yourself, while a buy-to-let mortgage is priced and underwritten on the basis that a tenant's rent, not your personal income, is the main thing paying it back.

This guide explains what "consent to let" actually means if your circumstances change after you have already bought on a residential deal, how buy-to-let products genuinely differ from residential ones, and what can go wrong if you end up on the wrong product for how the property is actually being used.

Consent to let

Consent to let is the arrangement most residential lenders offer if you already hold a residential mortgage and want, or need, to rent the property out, whether that is because you are relocating for work, moving in with a partner, or renting out a former home you have not yet sold. It is normally intended as a temporary bridge rather than a long-term solution, and typically comes with a few conditions worth knowing before you rely on it:

• You need to ask first. Renting out a residential-mortgaged property without informing your lender and getting permission is normally a breach of the mortgage conditions, even if you fully intend to move the property onto a proper buy-to-let deal eventually.
• It is often granted for a limited period, sometimes six to twelve months, after which the lender expects you to either move the property onto a buy-to-let product, sell it, or move back in.
• The rate may increase. Some lenders charge a slightly higher rate, or a one-off fee, for the consent to let period, reflecting the change in risk from an owner-occupied property to a let one.
• It is not automatic. A lender can decline consent to let, particularly if the property or your circumstances do not fit their criteria, in which case remortgaging onto a genuine buy-to-let product, or selling, become the realistic options.

If letting the property is likely to be a lasting arrangement rather than a genuinely temporary one, moving onto a proper buy-to-let mortgage sooner rather than later is usually the safer route, since consent to let arrangements are not designed to run indefinitely and a lender can, in principle, withdraw or decline to renew consent. Our guide to how buy-to-let mortgages work covers what that underwriting process actually involves once you make that move.

BTL product differences

The core difference between a buy-to-let mortgage and a residential one is what the lender actually assesses when deciding how much to lend and at what rate:

• Rental income drives the lending decision, not your salary. A residential mortgage is assessed mainly on your personal income and outgoings. A buy-to-let mortgage is assessed mainly on whether the property's rent clears an Interest Coverage Ratio (ICR) test, calculated at a stressed interest rate well above the actual pay rate, with your personal income treated as a secondary factor.
• Deposits are typically higher. Buy-to-let deposits commonly start at 25% of the property's value, capping standard lending at around 75% loan-to-value, compared with the 5% to 10% deposit sometimes available on a residential purchase. Our understanding LTV on buy-to-let guide sets out the typical ranges and why 75% remains a common ceiling.
• Most buy-to-let mortgages are interest-only. The monthly payment covers interest only, with the capital due at the end of the term, usually from a sale or remortgage, which keeps monthly cashflow lower but means the loan balance does not reduce on its own the way a residential repayment mortgage's does.
• Rates and fees tend to run higher. Buy-to-let products generally carry a slightly higher rate and, often, a higher arrangement fee than an equivalent residential product, reflecting the different risk profile lenders attach to rental property.
• Existing homeownership is usually expected. Most buy-to-let lenders want you to already own your own home, outright or with a mortgage, before considering a buy-to-let application, alongside a minimum personal income requirement on top of the rental assessment.

These differences exist because the two products are answering different underlying questions: can you, personally, afford this mortgage, versus can this property, through its tenant's rent, afford itself. Trying to answer the wrong question with the wrong product is where problems start.

Risks of the wrong product

Using the wrong mortgage product for how a property is actually being used carries risk beyond a simple technicality, and it is worth understanding what can genuinely go wrong:

• Breach of mortgage conditions. Letting a property on a residential mortgage without consent, or without moving to a buy-to-let product, is normally a breach of the mortgage terms. A lender who discovers this can demand immediate repayment of the full loan, which is a materially worse outcome than the inconvenience of arranging consent to let or remortgaging properly in the first place.
• Invalid buildings or landlord insurance. Standard residential home insurance is generally written on the basis that the property is owner-occupied. Letting it out without switching to landlord insurance can invalidate a claim entirely, leaving you personally exposed for damage or liability that would otherwise have been covered.
• No safety net if something goes wrong with the tenancy. A residential mortgage lender's terms are not built around the possibility of a tenant falling into arrears or a dispute arising. Being on the wrong product at the point something goes wrong adds an extra, avoidable complication on top of an already stressful situation.
• A worse deal than you needed. In the other direction, taking out a full buy-to-let product for a property you only intend to let briefly, when a straightforward consent to let arrangement on your existing residential deal would have covered the same period more cheaply, can mean paying buy-to-let rates and fees for longer than necessary.

A worked example shows how this plays out. A homeowner relocates for a two-year work contract and rents out their home rather than selling it, without telling their lender. Eighteen months in, a leak damages a neighbouring flat, and the insurer discovers the property has been let on a residential policy, voiding the claim. Separately, because consent to let was never sought, the mortgage lender is technically entitled to demand full repayment, though in practice most lenders would rather move the borrower onto a proper arrangement than call in the loan. Either outcome, an invalid insurance claim or a demand for repayment, is a far worse position than the ten-minute phone call to the lender would have cost at the outset. Our guide on how much deposit you need for buy-to-let is worth reading before assuming what a proper buy-to-let remortgage on the same property would actually require.

How Property HQ helps

Property HQ logs each property's mortgage type, lender and any consent to let or fixed-rate end date in one place, so you always know which product a property is actually on, and when it is time to review whether that product still matches how the property is being used.

Disclaimer

This guide is general information for UK landlords, not mortgage advice. Mortgage terms, consent to let policies and lending criteria vary by lender - speak to your lender or a qualified mortgage broker before renting out a residential-mortgaged property or applying for a buy-to-let mortgage.