Barn Swallows starting a nest

Its not legal to destroy a birds nest while its started building, So i'd suggest you leave it be.
Swallow numbers are down from recent years, Any chicks they do have is only a good thing to add to the numbers.
You are very lucky to have these birds nesting so close to you, I'd personally love that!

Can't you put something down under the nest so any droppings land on that and not your parcels? 
Its your garden so you are probably going to do what you like but if you can not disturb the nest and let it be you'd be helping these birds and there numbers.

Massive storm over greece in april killed alot of swallows and swifts coming back over to breed. 
You can read more about that here - www.dailymail.co.uk/sciencetech/article-8307451/UK-swallow-swift-sightings-fall-freak-storm-April.html