You can sell a house without a boiler installation certificate if the buyer agrees to take out indemnity insurance and if the local authority haven't been informed of the installation. Most buyers won't take on the risk of a boiler that hasn't been registered at Gas Safety or at Building Control. The cost to get the works signed off with a Gas Safety Engineer start from £200, however these costs can escalate if the boiler cannot be signed off in its current condition/position/installation.