Those seeds were both in the same pot - thought they might be root bound and once fairly well established decided to separate them into two pots - one plant's leaves turned brown quite quickly and now some four/six weeks later now the other plant's leaves are nearly all brown - I've sprayed them daily with good filtered structured water - and they were in a seasol indoor plant soil - I've kept them in the sunniest spot in our apartment and even covered them with a large plastic bag which I thought would give them a moist more tropical atmosphere. I've no idea why they both looking as if they are dying - but have wondered if it's just a winter issue here in Australia and the coming spring will help them grow again - any advice or help would be so greatly appreciated - I've no idea what to do with them.
When you separated those, the destruction of roots may have sent them into shock, and the one that turned brown first must have been worse. The bags may have given them too much humidity. I only see bags suggested when one is rooting a cutting. Usually plants bounce back from shock of repotting though. If you think they are still alive, leave off the bags, and water only when the soil is dry. Leave it in the sunny spot, and hopefully it will grow new, green leaves.
Thinking of doing a building project but there is a lychee tree in the area. Is the lychee root system a problem? Would it be possible that this would create a problem in the foundation of the building?
Lychee is a medium sized tree with a canopy spread of 20 to 30 feet. The potential damage from roots is low so you should be able to construct a building nearby. It still should be 15 to 20 feet from any foundation. A publication included below says lychee should be planted 25 to 30 feet away from any building on a house lot.
https://selectree.calpoly.edu/tree-detail/826
https://www.gardeningknowhow.com/garden-how-to/design/lideas/foundation-plant-spacing.htm