Hi Fieldfare. How lovely to hear from someone who wants to accommodate the birds. When we first moved here we displaced some swallows, which still try to build in the house if we leave the doors and windows open when they are choosing a site. They happily moved into the outbuildings and the hen houses - not so happy about that because I think they bring red mite with them, but there's no keeping them out - they go in the popholes so we have given in and made permanent access slits above the doors, which have the secondary purpose of aiding ventilation. Their nests need to be safe from predators such as magpies which will pull down the nests and eat the eggs/chicks. Of course you have to do the work during the time they are in Africa or they will start to nest in your building site.
We also displaced blue tits when we renovated the granny flat, and it took a while for them to adopt the nest box we put up for them instead. Overall though by planting hedges and a small plantation (for cover, nest sites and calm winds for insects), and making sure there are plenty of flowers to attract insects, provide nectar etc,and plants with seed heads and berries, we have increased the numbers of birds manyfold.
This year, although they started late, we had at least 9 swallows nests, compared with about 3 16 years ago, plus the martins this year. Swallows seem remarkably tolerant of human activity and either just watch you from above, or if you are being annoying they will dive bomb your head to get their point across.
You can buy premade swallows nests which you fix where you want them to nest, andf I presume you can also get martins nests too, which are a more enclosed shape. Ah but I see from SuzieQs reply that they are not too fond of them. This spring when it was so dry, swallows had little to make their nests from, with no wet mud around, so ours made less strong nests from hay and other things which other birds use. I thought they might have liked the preformed nests then.