If you’ve done works without council approval then there’s a whole bunch of issues which can emerge, but, you wouldn’t be the first, and if you were thinking of selling, plenty of houses are sold all the time with “unauthorised improvements”.
However in every contract of sale there are certain statutory warranties which apply “except as disclosed in the contract”. One of the statutory warranties is:
there is no matter in relation to any building or structure on the land (being a building or structure that is included in the sale of the land) that would justify the making of any upgrading or demolition order or, if there is such a matter, a building certificate has been issued in relation to the building or structure since the matter arose…
Generally speaking, home building works which require consent when none was obtained is likely to be something we would justify your local council making an upgrading or demolition order.
So, it’s crucial that such improvements are properly and clearly disclosed in the contract. However, mere disclosure will often not be enough and your lawyer will need to ensure that the purchaser can’t make any claims in relation to the unauthorised improvements.