A clueless question

Wow-- US tax law is scary. There seems to be so much that you can overlook or not anticipate. Here's one question that comes up now that I (US citizen) and my husband (French citizen, both of us currently UK residents) are considering **maybe** moving back to the US: what would our tax liability be on the sale of our house here? We paid stamp duty when we bought it-- would HMRC take anything more out upon its sale? And how does capital gains tax work? Would we owe something on the sale of our UK house to the IRS if we were in the process of finalizing my husband's green card and moving back to the US?
It's all baffling. And like I said, more than a little scary.

If the house is your main home you probably won't have to pay any UK capital gains tax because of "Private Residence Relief". For US taxes, if you used the home as your principal residence for at least two out of the five years before the sale of the home then $250k of capital gains ($500k if your filing status is married filing jointly) can be exclude from your tax.