Hannah of https://www.mccartanlettings.co.uk/ explains how the new stamp duty changes affect landlords.

Stamp duty percentages for second properties changed on April 6th 2016. This directly affects landlords who are looking to purchase in the future as there s now a 3% surcharge on top of the original stamp duty price.

Whereas before there was no stamp duty payable for properties up to £125, 000 from now on all properties from £40, 000 and up to £125, 000 will be charged the 3% stamp duty and all properties £125, 000 and above will be charged the original 2% + the new 3% giving a total of 5% stamp duty on properties above £125, 000.
So this has a dramatic effect on a landlords cash flow when purchasing investment properties.

The flip side is it also affects landlords who wish to move home because their home is still essentially a second property they will have to pay an extra 3% stamp duty on their next home.
This could lock landlords into their original homes and they may not now be in a position to move due to these extra costs.

So when budgeting to buy your next investment property you must be looking at that extra 3% and making sure you have that extra cash available.