Of the landlords who intend to sell property, the 45% of plan to let go of individual flats and apartments, while a third are looking to sell terraced homes.
The flood of homes is good news for first-time buyers but bad for renters, the trade body said.
Just 7% of landlords planning to sell are intending to pass on to property to other landlords.
Richard Lambert, chief executive of the National Landlords Association (NLA), said: “These findings sound like positive news for potential new homeowners, but the reality is not everyone wants, or is in a position financially, to buy.
“In fact, if all these homes are sold as planned then it will lead to a significant fall in the supply of property available to those who choose to rent or have no other option but to rent.”