The lender, which is the intermediary lender of The Co-operative Bank, will also no longer accept applications for those wanting to convert their existing residential property to a buy-to-let property.
Platform will only allow those currently on a buy-to-let deal to remortgage to a buy-to-let product.
The maximum number of buy-to-lets that an individual can have with all lenders, including Platform will be limited to five.
In September Lloyds Banking Group limited buy-to-let property portfolios to a maximum of three properties or £2m worth of lending – whichever is exceeded first.
Platform’s maximum property exposure is three properties or £1m.
A spokesman for Platform, says: “Platform remains committed to the buy-to-let market as well as to mainstream lending and we will shortly be extending our range of buy-to-let products.
“We have a wealth of experience and expertise in buy-to-let lending and we’re very clear about the area of the market that we want to focus on. This is reflected in our criteria.”