You can get a bill a year later, but not for extras under the contract, which normally should have been assessed under the main run of the contract before handover and resolved in the Final Account.
Under several forms of contract, retention is held on the builder to ensure he completes the works and doesn't leave work undone, or doesn't pay enough attention to finishing prior to handover.
The building when handed over should be fit for purpose within the terms of the contract, and the retention mechanism is in place to ensure this occurs.
The Certificate of Practical Completion issues at the end of the Works, and the Penultimate Certificate for Payment releases the First Moiety of Retention Monies, if the main work is completed to the Architect's satisfaction, saving and excepting minor /snag items.
The Defects Liability commences on or around that date and usually runs for a year.
At the end of this year the builder is entitled to seek release of the Second Moiety of retention.
This is provided that all the snag items have been attended to and any defects arising duing the year have been addressed.
So you shouldn't get a bill for extras.
Unless in addition to him addressing the snag items and defects arising, you have asked your builder to do more works to the building not covered by the previous contract.
Or...
Unless you didn't have professionals looking after your interest and no final account was done.
Then you may have a bit of a mess to sort out and I strongly suggest you talk to a Quantity Surveyor, who is skilled to handle such disputes.
HTH
ONQ.
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