The high street bank says this means that existing customers can reserve a new rate 120 days before their current deal comes to an end, an increase from 90 days.
The bank says: “At a time when rates are rising, this additional month in which customers can secure a rate could make an important difference in their monthly mortgage payment.”
The move comes amid rising rates and inflation, which has caused lenders, not only to lift the cost of their loans, but to offer fewer products for limited periods.
The average time a mortgage product remains on the market is currently 17 days, according to data group Moneyfacts data last month.
It adds that August began with 4,407 mortgages on offer, which is 149 fewer than were available at the start of July.
HSBC head of buying a home Michelle Andrews says: “In recent years we have seen and enjoyed low mortgage rates, but over the last few months interest rates have been increasing, for a number of reasons.
“As rates have recently been creeping up, we know that securing a new deal before rates potentially change again is important in terms of cost-management but also for peace of mind for our customers.
“We know that many homeowners will be looking to review their mortgage deal earlier than usual. By extending the window w. customers can select a new rate with us, this could help customers during what could be a stressful and challenging time for them.”