As per IAS 16, an item of property, plant, and equipment should be recognized as an asset if and only if it is probable that future economic benefits associated with the asset will flow to the entity and the cost of the item can be measured reliably.
Any expenditure incurred that meets these recognition criteria must be accounted for as an asset. The Standard makes reference to individually insignificant items that can be aggregated. Subsequent costs should add to the utility or useful life of the asset.
In this case the professional fees paid for the extension of the building should be aggregated to total cost of extension and capitalized.