Compare like with like: the net area in the sale agreement and its plan is the statutory baseline. A brochure's gross area, an internal room measurement and the registered net area may describe different things. Check how balconies and other spaces are treated.
Under that rule, a shortfall of exactly 5% is not the same as one exceeding 5%; compensation is calculated by reference to the agreed price. Have the survey and calculation checked rather than assuming a percentage from an informal measurement.
An area claim is not automatically a right to cancel the purchase. Material specification changes, misleading representations or other contractual rights need separate assessment. A resale or a property in a different legal regime should not be treated as identical to an off-plan developer transaction.
A practical next step
Obtain the signed contract, attached plan, registered area and an appropriate survey record. Raise the discrepancy in writing before signing a final settlement that may affect your claim.
Official sources
Executive Council Resolution No. 6 of 2010 — net area and compensation, Article 13
Know Your Rights — property purchase checks