Metro Bank is offering first-time buyers a 100 per cent mortgage, meaning they can get on the property ladder without needing to save a deposit. However, they will need their parents to be added to the mortgage in order to get it. This is because it is a 'joint borrower sole proprietor' mortgage, meaning a buyer can add a second person to their mortgage application without them owning the property. The joint borrower's income is also counted when the bank assesses whether the buyer can afford the mortgage, which usually means the buyer can borrow more.However, it also means that, if the first-time buyer can't pay the mortgage in future, the parent will be responsible for doing so. Any missed payments could affect the parent's credit file as well as the first-time buyer's, and brokers have warned this could affect their ability to borrow and retirement plans. No deposit: Home buyers can get a loan that'll cover all of the purchase price with Metro BankMetro says eligible buyers will be able to borrow up to 100 per cent of the value of a property, providing they have an immediate family member such as a parent to act as the joint borrower in the event of a missed payment or change in financial circumstances. The mortgage rate is fixed for five years, and the buyer must take the mortgage with a minimum repayment term of 35 years.They can put down a small deposit of less than 5 per cent of the home's value if they wish, and this would reduce the amount they repay overall. If someone has a deposit of 5 per cent or more and doesn't need family help to pass affordability checks, they are better off looking for a different mortgage as they can get a far lower interest rate. Watch out for high rates While there is no product or valuation fee, the interest rates are high compared to other small deposit mortgages. Someone looking to lock in the deal with a mortgage covering more than 95 per cent of the purchase price will have to settle for 6.99 per cent rate.On a £200,000 mortgage that would mean paying £1,277 a month for the next five years.That's a lot more expensive than what someone with a 5 per cent deposit can secure with other lenders across the market at present.For example, Skipton Building Society currently offers a 4.98 per cent five-year fix for those with a 5 per cent deposit, with no extra fees. However, this doesn't allow buyers to add their family to the mortgage, so they'll need to be able to pass the affordability tests based on their income alone. On a £200,000 mortgage with a 35 year repayment term that would mean paying £1,007 a month for the next five years. A £270 monthly saving.Aaron Strutt, mortgage broker at Trinity Financial, says: 'The rate is not amazing at 6.99 per cent fixed for five years, but if the product allows often younger people and family members to club together and buy the property they want it could be a good option.'If you can get a small deposit together to qualify for a cheaper mortgage elsewhere it may well be worth doing.'However, Strutt says there are some features that might make it appealing to some.For a start, it allows for a maximum mortgage size of £675,000, which is generous given that applicants potentially have no deposit.It also allows up to four people on the mortgage, with all four incomes being considered for mortgage affordability purposes.The mortgage can be up to 95 per cent of the purchase price with non-family supporting borrowers, and up to 100 per cent of the price where all borrowers are immediate family members.Properties above commercial premises or new builds are not accepted.Strutt adds: 'If you are desperate to get on the property ladder and it's cheaper than renting, it may well be worth considering.''At a time when younger people need more help to buy homes this is a welcome addition, but it isn't cheap.'Other mortgages with family help There are some other similar deals currently on the market.Most require either a guarantor, some form of collateral security (for example from a parent's property) or a cash deposit to be held with the lender for a period of time.The Barclays Family Springboard Mortgage allows buyers to borrow with no deposit if a family member or other benefactor will provide 10 per cent of the property price as security and keep it in a Barclays savings account. The helper's money is locked away for a set period and can be returned with interest if the borrower keeps up repayments.Family Building Society's Family Mortgage also allows borrowers to buy with no deposit if family members provide security instead. They need to hold savings with the building society, or agree to a charge being put on their own home. The security, plus any buyer deposit, must equal 20 per cent of the property value. Buyer boost: Joint borrower, sole proprietor mortgages can help young people get on the property ladder - but their parents will need to agree to be on the hook if they miss paymentsJoint borrower sole proprietor mortgages are also available with Skipton, Barclays, Newcastle Building Society and Cambridge Building Society. These are not always 100 per cent mortgages, but they can help buyers borrow more.There are also non-family 100 per cent mortgage options, such as Skipton's track record mortgage. This is aimed at those who can prove a solid history of making rent payments. Parents urged to understand the risks Mortgage brokers say families considering a mortgage like this one should consider getting financial advice as the parents are taking on a significant commitment.Doug Miller, director at Bath-based Lansdown Financial Services, said: 'We have seen a significant increase in demand for joint borrower, sole proprietor mortgages as property prices continue to rise and affordability has become stretched.'A 100 per cent joint borrower sole proprietor option will undoubtedly open the door for some buyers who would otherwise be locked out of the market, but it also increases the importance of robust affordability assessments and clear advice for everyone involved.'Parents or family members acting as joint borrowers are taking on a significant financial commitment, so it's essential they fully understand the risks as well as the opportunities.'Samuel Mather-Holgate, managing director at Swindon-based broker Mather and Murray Financial, added: 'These mortgages can be a useful way of helping first-time buyers onto the ladder, and we do see interest in them, especially where parents want to support without being added to the property title. But 100 per cent lending makes the risk much sharper.'The parent is not just offering moral support, they are jointly liable for the mortgage. If payments are missed, it can affect their credit file, future borrowing and retirement plans.'It is good to see innovation, but this needs very careful advice. The big question is not just whether the buyer can get the keys, but what happens if rates rise, income falls, or the parent later needs borrowing of their own.'Best mortgage rates and how to find them Mortgage rates have shot up again due to inflation triggered by the conflict with Iran reversing hopes that the Bank of England would cut rates. This means those remortgaging or buying a home face higher costs.That makes it even more important to search out the best possible rate for you and get good mortgage advice, whether you are a first-time buyer, home owner or buy-to-let landlord.This is Money's partner L&C can help you with its fee-free mortgage service.> Compare mortgage rates> Find the right mortgage for you To help our readers find the best mortgage, This is Money has partnered with the UK's leading fee-free broker L&C.This is Money and L&C's mortgage calculator can let you compare deals to see which ones suit your home's value and level of deposit.You can compare fixed rate lengths, from two-year fixes, to five-year fixes and ten-year fixes.If you’re ready to find your next mortgage, why not use This is Money and L&C’s online Mortgage Finder. It will search 1,000’s of deals from more than 90 different lenders to discover the best deal for you.> Find your best mortgage deal with This is Money and L&C Mortgage service provided by London & Country Mortgages (L&C), which is authorised and regulated by the Financial Conduct Authority (registered number: 143002). The FCA does not regulate most Buy to Let mortgages. Your home or property may be repossessed if you do not keep up repayments on your mortgage.