I would love to have housemartins but I think my house is a little too low, but I do have swallows.
For many years there was only one nest each year, but after a while by carefully making more places they might like to build when I was doing any property improvements I built up to 8 one year, though our current rainy weather has knocked them back
They often try to build in the porches right over the doorways and I tried various deterrent methods each year before they started building (with no avail I might say) Then hit on a brainwave and got a hanging basket coconut type liner cut it up and stapelled it into "swallow nest" shapes which were then fixed in locations I DID want them. INSTANT success. They moved into the first one and started nest making the same day it was fixed up. They also appreciate a mound of small feathery fluffy stuff nearby for nest lining material
I know you can buy housemartin preformed nests. Perhaps you could fix a few of those in the least troublesome position and see what happens. Though it would be worth noting the aspect of their preferred locations in case you fix them where it is too hot-cold-sunny-windy etc for their precise breeding requirements.
House where housemartins live are happy homes - old proverb