New piece of land for garden extension

Get a load of well-rotted manure and some compost (? farmer, riding school, local council, cheap bags full). You will need it for the borders if you have clay. It will help make the soil more workable later.

I once recommended adding coarse sand and grit to clay and someoneelse wrote in to say you shouldn't do that, it would make it worse. But I didn't really understand as it made the patch of soil I added it to much better. Perhaps you would need too much for a whole lawn. See if anyone else answers. Manure in th beds will feed plants as well.

How about some photos of the land?

Adding grit and coarse sand, as well as as much compost & manure as you can lay your hands on, will always improve clay soil, so would strongly suggest you do that. If the land has a history of becoming waterlogged though, it would be worth getting a professional to look at it with a view to perhaps getting some land drainage pipes laid before you get started.  However, with all of the rain we've had over the last few months, if the land is not waterlogged right now it probably doesn't need serious extra drainage and adding grit/compost should help a great deal.