The ROI estimate shown within the Property Tools browser extension uses the asking price as the purchase price and uses a median of local comparable rental property rates as the rent. From these, PaTMa calculates the minimum amount of potential cash investment (covering SDLT, conveyancing, mortgage fees and deposit). The deposit is calculated as the minimum possible with 75% LTV or less and to meet a rental cover test of 150% at 5.5%. (These exact restrictions may change over time as market conditions change.)

The resulting mortgage interest costs are obviously allowed for in the ROI calculation along with 15% of rent for maintenance and voids.

These are basically the default values for the PaTMa buy-to-let profit calculator . You can scroll down on that page you to see lots more detail on how things are actually calculated.

These figures are only going to be an approximation for most people, hence there's a "save as a prospect" link in the information panel provided by the extension on each detailed property page, or you can add prospects directly from the Prospector pages in PaTMa. Adding a prospect will open a form (with lots of pre-filled details if clicking from a listing). You can customise your expected purchase price, any refurbishment costs, the expected rent and all the ongoing costs (agent fees, ground rent, etc) so you can get a fully customised ROI forecast. Once you've saved your first prospect you can also adjust your investment scenario (cash available, LTV limit, etc) for better figures.

Saving a prospect from a portal listing also means you'll get an email if that listing changes price or becomes unavailable (or re-available).