If you dream of helping your child on to the property ladder but don't have buckets of cash to give or lend them for a deposit then you might consider becoming a 'booster' on their mortgage application. Growing numbers of lenders are launching products that allow you to help someone buy a property by adding your income to theirs for the purposes of affordability calculations.

These so-called 'joint borrower sole proprietor' or 'booster' mortgages have been typically offered by specialist building societies and now they're popping up on the high street. NatWest has launched its version, the 'NatWest Family Backed Mortgage' and a similar product from Barclays is called 'Mortgage Boost'.

Here's how they work – and the pitfalls you need to watch out for.

What is a booster mortgage?

Around half of first-time buyers get financial support from family towards a deposit, figures from estate agency Savills suggest. They received £55,572 on average in gifts and loans last year.

But many parents do not have that kind of money to hand over. The income boost mortgage provides an alternative way to help out.

They allow parents or another family member or friend to apply for a joint mortgage with the buyer.

The advantage is that the buyer should be able to take out a larger mortgage. Take, for example, a first-time buyer earning £28,000 a year with a ten per cent deposit. If buying alone, they would have a budget of £140,000 to buy their first home, assuming they borrow a typical four and a half times their annual earnings. That is unlikely to be enough to buy a first home as the average house price now sits at more than double that at £292,781, according to lender Halifax.

However, if a family member or friend with an income of £45,000 with no mortgage of their own agreed to be named on a booster mortgage, the combined income would be £73,000. That would bring the buyer's budget up to £365,000, says NatWest – which is 4.5 times the combined income with a ten per cent deposit.

Some lenders, such as Skipton or Family Building Society, allow up to four borrowers on the mortgage – two borrowers and two boosters.

Crucially, the boosters are not named on the property deeds. That means they won't inhibit the buyer's ability to benefit from any tax breaks available for first-time buyers. First-time buyers do not typically pay stamp duty on homes under £300,000, but if there is a buyer on the deeds who s or has previously ed a property, they pay stamp duty on properties that cost above £125,000.

Lenders typically allow boosters to be aged up to 75 or 80 by the end of the mortgage term, which may make it more difficult for older parents to be on the mortgage. Boosters could be no older than 55 if the first-time buyer wanted to take out a 25-year mortgage.

Skipton Building Society, which offers this type of mortgage called 'Income Booster', says applications are coming from across the UK, but particularly in higher-value areas such as London.

Who pays the mortgage?

Mortgage brokers say in most cases the buyer pays the whole monthly mortgage repayment so the booster doesn't have to pay anything. The monthly mortgage payments are often less than what the buyer was paying in rent. In some cases the family pay some towards the mortgage each month, but brokers say this is rare.

While the main borrower tends to make the repayments, all borrowers named on the mortgage are jointly liable. That means that if the buyer doesn't pay, the boosters have to. In the event of either party's death, the remaining applicant(s) remain responsible for the full mortgage, just like with any standard mortgage.

Because the income booster has an obligation to the repayments, it's recommended that everyone involved receives legal and financial advice to avoid misunderstandings and to provide clarity should the worst happen.

When does the agreement end?

NatWest says the booster can be removed at any time. When the non-ing borrower leaves the mortgage, it becomes a standard sole mortgage.

Are there other advantages?

Richard Dana, of mortgage broker Tembo Money, says some customers prefer the income boost option to handing over a cash gift if they have reservations about their child's partner.

When a cash gift is handed over, the giver has no control over what happens to it. So, for example, while they may make the gift to their child, there is a risk it could be split with their partner should they divorce.

Can others benefit?

Lenders say that sometimes younger people use booster mortgages to help their parents buy a home. Family Building Society says it provided this type of mortgage to an 83-year-old who needed to remortgage the home she'd lived in for 22 years. Her daughter was able to be an income booster.

what about the downsides?

The booster takes on shared responsibility for the loan, but does not benefit by gaining any ownership or equity in the property. If the borrower cannot keep up with the repayments, the booster will have to step in.

Dr Alla Koblyakova, an expert in mortgage finance at Nottingham Trent University, says the booster may find it difficult to extricate themselves from the arrangement and pursue their plans. It could be problematic for young boosters as it may limit their ability to buy their property.

They also wouldn't be able to benefit from first-time buyer stamp duty relief to buy their property because they are already named on another mortgage.

What are the alternatives?

Other options that do not involve gifting or lending a deposit include a family offset mortgage, where a parent or other family member links their savings to a child's mortgage. The total mortgage debt is offset by the savings and interest is only charged on the balance. It reduces the amount of interest that the child needs to pay.

Another option is shared ership, a scheme where a borrower can buy a share of a property and pay rent on the rest.

How do rates compare?

Interest rates for a booster mortgage are similar to other deals at around 5.5 per cent for two- or five-year fixed deals.