This question addresses two issues: houses that look unfinished and houses that are unfinished and maybe have stood so for a long time.
In the first instance the houses may LOOK unfinished because iron bars stick on top of the roofs.
This is absolutely not to avoid paying property tax (as the legend has it) but to simplify the building of another floor in some distant future.
A house could also BE unfinished because the owners are waiting for finances to continue with the building.
Until a few years ago Greece had a very high inflation rate and it was wise to put any money you had into a house and build as money became available.
Although the inflation is now on par with the rest of Europe it seems that this habit is still surviving.