Interest-Only to Repayment: Calculator and Transition Strategy

The Payment Shock

Interest-only payments cover only the monthly interest, leaving the capital untouched. £200,000 at 4.30% is £717 per month regardless of term. Repayment mortgages amortise both interest and capital, so the payment depends on term. Over 25 years: £1,088. Over 20 years: £1,247. Over 15 years: £1,506. Over 10 years: £2,051. The shorter the remaining term, the more capital must be packed into each monthly payment, and the steeper the cliff.

The payment jump at the switch is usually 50% to 180% depending on remaining term. Borrowers who have been on interest-only for 15 years and have 10 years left to the original term face the biggest jumps because the remaining term is short and amortisation is fast. Plan the switch as a major household-budget event — review subscriptions, insurance, car finance and other commitments ahead of the change rather than firefighting afterwards.

Loan	Interest-only monthly	Repayment over 25y	Repayment over 15y	Repayment over 10y
£150,000 @ 4.30%	£538	£816	£1,130	£1,538
£200,000 @ 4.30%	£717	£1,088	£1,506	£2,051
£300,000 @ 4.30%	£1,075	£1,632	£2,260	£3,077
£450,000 @ 4.30%	£1,613	£2,448	£3,390	£4,615

The FCA Maturity Rules

Since the Mortgage Market Review in 2014, lenders must engage interest-only borrowers at least two times before maturity to understand the repayment plan. Since Consumer Duty (2023), this has intensified: lenders must identify and support borrowers whose plans are inadequate. The FCA's 2022 and 2024 reviews of interest-only portfolios estimated that 20-25% of outstanding interest-only loans had inadequate or no credible repayment plan.

If your original plan (endowment, ISA, investment property sale, lottery) is not tracking, the lender will contact you 24 to 36 months before maturity. Your options are: switch to repayment, extend the term, sell and downsize, use retirement interest-only (RIO), or refinance to a specialist lender. Doing nothing is not an option: at maturity, the full capital is due.

The FOS has upheld complaints where borrowers were not given adequate warning or support, and lenders have made settlements in the thousands. Borrowers approaching maturity without a plan should engage the lender early rather than wait.

Worked Example 1: Full Switch at Remortgage

Martin is 55 with a £160,000 interest-only balance, 10 years to maturity, on a 4.55% product expiring in 2026. He has no repayment vehicle. At remortgage he switches to full capital repayment over 10 years. New rate available: 4.30% (75% LTV on £250,000 property).

Old monthly payment: £607 (interest-only). New monthly payment: £1,641 (10-year repayment). Jump of £1,034/month — massive. Over the 10 years, total paid is £196,920 (vs continuing interest-only and owing £160,000 at maturity). Martin ends mortgage-free at 65, which aligns with his retirement.

Affordability at the new payment: Martin's household income is £58,000 gross, giving monthly net of about £3,650. At £1,641 mortgage payment plus £1,200 household costs, he has £809 residual — tight but feasible. Most high-street lenders would approve the switch with documentation of stable income. Extending to 12 or 15 years would ease the monthly payment at the cost of carrying mortgage debt into retirement.

Worked Example 2: Partial Switch

Diane has a £250,000 interest-only balance with 12 years remaining. Her repayment vehicle is a £150,000 ISA portfolio projected to grow to £180,000 by maturity. Shortfall: £70,000. She remortgages to a part-and-part structure: £180,000 stays interest-only (matched to ISA); £70,000 moves to repayment over 12 years.

Calculation: interest-only portion at 4.30% = £645. Repayment portion £70,000 over 12 years at 4.30% = £645. Total new payment: £1,290. Old pure interest-only on £250,000: £896. Jump of £394, much more manageable than a full switch which would have been £2,322.

Partial switches are available at Halifax, NatWest, Nationwide (above 50% LTV residential) and most BTL lenders. They work well where the existing repayment vehicle is genuine but incomplete. The FCA encourages this structure for borrowers with verifiable ISA or pension assets; it produces better outcomes than a full switch for those with stretched affordability.

Worked Example 3: Term Extension + Repayment

Roger is 62 with a £120,000 interest-only balance, 3 years to maturity, and no credible repayment plan. A full switch to repayment over 3 years would cost £3,569/month — clearly unaffordable. His options: extend the term to give amortisation time, or sell and downsize.

At remortgage, he extends to a 15-year term with full repayment. New monthly payment over 15 years at 4.30%: £904. His income (teaching pension plus part-time consultancy) is £34,000 gross, so £2,120 net monthly. At £904 mortgage plus £1,000 living costs, he has £216 residual. Affordability is tight but passable under the lender's "retirement income" approach.

The 15-year term means Roger will have mortgage payments until 77. Lenders' maximum-age rules vary: Halifax maximum age is 80, Nationwide 75, Santander 75. The extension is available at Halifax but not Nationwide. Alternative: downsize to a £200,000 property, clear the mortgage from equity, and remove the burden entirely.

Retirement Interest-Only (RIO)

RIO mortgages are a specific product type for over-55s: interest-only payments continue until the borrower dies, sells, or moves into long-term care, at which point the capital is repaid from the property sale. They sidestep the "repayment plan" requirement because the end event (death/sale) is the repayment plan.

RIO rates in 2026 are higher than standard remortgages: typically 5.0% to 6.5% vs 4.3% to 4.6%. They are underwritten on current income (usually pension income), not on 25-year forecasts. Major RIO lenders include LV=, Hodge, Vida Homeloans and Nationwide's retirement range.

For borrowers approaching interest-only maturity with adequate equity but inadequate repayment plan, RIO can be a clean solution: keep living in the house, keep paying interest, clear the capital from the estate. The trade-off is that your estate inheritance is reduced; and RIO has higher rates than conventional products, so over 20 years the cumulative cost is meaningful.

When to Switch: The Timing Question

Switch early if you have the income to absorb the payment increase. Every year of interest-only that could have been repayment adds unnecessary interest to the lifetime cost of the loan. A £200,000 loan that spends 5 extra years on interest-only before switching pays approximately £36,000 of extra interest that a full repayment would have avoided.

Switch late if income is tight now but strong later (nearing bonus maturity, imminent inheritance, business crystallisation). The interest-only period buys time. But the switch must eventually happen; paying interest-only right up to maturity means a full capital bill of hundreds of thousands with no amortisation.

The FCA's Consumer Duty requires lenders to engage proactively; use that engagement. Your current lender will usually offer a product transfer to a repayment structure at their standard rates; if the deal is competitive, take it. If not, remortgage to a lender with a better interest-only-to-repayment track (Nationwide, Halifax, Barclays are all reasonable).

Sense-Checking Your Own Numbers

Three checks for anyone on interest-only approaching maturity. Check one: confirm the balance. Interest-only loans do not amortise, so your balance is still whatever you originally borrowed (minus any overpayments). This seems obvious but many borrowers misremember the figure. Pull the latest annual statement and note both the outstanding capital and any early-repayment-charge schedule still in place.

Check two: stress-test the repayment payment at 6.5% or 7.0%, not today's rate. Rates can rise during the remortgage process; you want certainty you can afford even under a stressed scenario. On a £200,000 loan over 15 years, the stressed payment is £1,744 vs £1,506 at current rates. Factor in likely future rises in council tax, energy and insurance too — these compound with mortgage changes to tighten monthly cash flow.

Check three: reconcile the repayment plan, if any, with current market prices. Endowment policies projected 25 years ago are now maturing at 60-80% of projected values because actual investment returns under-shot the optimistic projection assumptions used in the 1990s. ISA portfolios with heavy equity exposure can be volatile; get a current statement and assume a conservative 3% real return for any future growth. Any plan that leaves you with a shortfall at maturity will need the partial or full switch to close the gap.

Once the three checks are complete, draft a one-page maturity plan: current balance, months to maturity, expected repayment-vehicle value, shortfall (if any), preferred remedy (full switch, partial switch, term extension, RIO, downsize) and the target date for action. Share this plan with your broker or adviser. A written plan gets much better results from lenders than a reactive conversation 6 months before maturity, because the lender can see you have engaged seriously rather than avoided the problem.