House Doctor: We need a bigger house but can we afford the mortgage into our retirement?

Question: We want to move to a bigger home with enough room for our teenage children, but we'd need to extend our mortgage to do so. So after paying it off since 1996 – and currently with 11 years left to run – our new affordable deal would see our home loan stretched to 18 years.

This would mean I (and my wife, probably) would be making repayments well into our retirement. Is this too much to take on in old age? Currently in our late 40s, we both plan to work until our mid-sixties.

Roger Eddson, Liverpool

Answer: A twilight cruise, smiling strolls on sandy beaches, dozing in deckchairs – a frantic scrabble to meet a mortgage payment? Whatever you think retirement might look like, the last vision is the least attractive. Repaying a mortgage from a monthly pension can take a heavy toll on your finances.

Home loans are most easily afforded by working adults simply because employment allows you, within reason, to earn a rising – and flexible – income. Whether it's through a move, better qualifications, promotion, overtime or a career change, a better-paying job allows you to better shoulder financial burdens like mortgages.

In retirement, for most people, your income is fixed – and usually at a much lower level than when working. So unless you've already laid fruitful pension plans and are very confident that your old age won't be a spartan struggle, it might be difficult to meet hefty commitments.

And while you might be cheerfully upbeat about such a future scenario, that isn't enough: you're going to have to prove it to a lender or broker, warns David Hollingworth at broker London & Country.

"Lenders don't tend to like borrowers repaying once they're retired and so will want to see evidence that your finances will be robust enough to cover it," he says. You will need to provide proof of pension size projections, forecast savings and any other income streams.

Worryingly, recent research by property agent Savills suggested that greater numbers of homeowners could still be repaying mortgages in retirement. The credit crunch and subsequent fallout has choked off the usual lending to homeowners, forcing them to wait longer before moving up the ladder.

Traditionally, a seven-year gap marks the difference between buying a first property and making that first move up the ladder: now, however, the gulf has stretched to 12 years, Savills estimates.

That's not all: while you might both plan to work until you're 65, other obstacles may get in the way: ill health, lifestyle shift or even a simple change of heart.

"This might all seem like a gloomy prognosis," says Andrew Montlake of broker Coreco, "but don't despair: your current age and – if affordable – hopefully robust financial position should still make it possible; if your finances stack up, you won't have a problem."