As seller, you can sign the contract and then change the completion date afterwards.
Even as a buyer, you can try to renegotiate the completion date after signing the
contract, depending on circumstances.
It is a very stressful juggling act, but the ideal synario is
- You get a signed contract for the sale of your house
- You get your offer accepted to buy a house
- Negiotate a suitable completion date and juggle the two to happen on the same day
- If you can convince your lender and you can cope with the financial pain, a closed
end bridging loan can allow you to complete the purchase before the you complete
the sale of your own, which allows you a little more time to move
Good luck