Major works that, unlike the above, do not give rise to a new building but that takes place upon buildings that are already constructed, regardless of whether there have been partial demolitions or not. They can be of two types:

- Extension work:
When the already-constructed surface area is extended incorporating new structural elements. This increase can be carried out vertically without increasing the surface area occupied on the land (eg. adding a new storey) or horizontally, and in this case surface area does have to be extended upon the land.

- Alteration and/or refurbishment work:
When the constructed surface area does not vary but the building undergoes modifications that affect some structural element, or some facility that did not previously exist is added.