The buy-to-let market looks set to expand further with more than half of landlords planning to increase their property portfolios over the next six months.

According to research of 218 investors by specialist mortgage broker Mortgages for Business, landlord appetite for more purchases stems from the attractive yields available on residential investments.

More landlords plan to purchase houses in multiple occupation (26%) and multi-unit freehold blocks (16%). Fewer investors plan to buy semi-commercial property (11%) and commercial property (7%).

Just 6% of landlords plan to trim their portfolios over the next six months - the same proportion as six months ago.

Two thirds of landlords planning to purchase more property said they will need to refinance in order to do so. In total, 43% of landlords said they will look to remortgage in the first half of 2013, up from 36% from six months ago.

Even though 45% of landlords don't envisage growing their portfolio in the first half of 2013, one quarter of them still plan to remortgage.

However, 11% of the landlords who want to expand their portfolios won't be able to refinance because of lack of equity and the difficulty in securing a mortgage with an LTV of more than 75%.

More than three quarters of investors (76%) said that lenders should do more to help them get the finance they need. Some 45% of investors felt criteria should be eased, with more preference given to experienced landlords and a greater willingness to lend on more complex property types.

The research also found that 39% investors rely entirely on rental income, which illustrates the rise of the professional property investor.