Thank you so much for your quick answer! It helped me a lot! Your way of doing it is certainly simpler than what I was thinking (which included padding the remaining cells with some dummy instances etc. etc.)
I now implemented it and I would like to explain it for further references.
In order to let the "construction" of buildings, I made a class that is similar to a normal unit but when it is created and placed on the map, it does the following:
Uses the "bottom left" cell as the instance "anchor".
Cycles through the cells where the building is supposed to be. This is specially easy since we can just add one to the X and Y till we get to the height and width of the building.
Sets the cell type by using
Additionally, I had to shift the instance image. In order to do this one can add the "y_offset" to the image in the xml that describes the object. For instance:
<object id="beach_bar" namespace="http://www.fifengine.net/xml/rio_de_hola" blocking="1" static="1" anchor_x="500" anchor_y="500">
<image source="beach_bar/000.png" direction="0"[b] x_offset="0" y_offset="-110[/b]"/>
Once again, thank you helios for your help and I might come asking again soon