Say I build myself a faraday cage/wave screen around my house,
potentially resulting in poor nework coverage for my neighbours.
Questions about land property and constructing permits apart, can I be
sued for that? By the network operator? by the neighbours?
In most countries, the use of the radio spectrum is regulated (who may send what on which frequency, at which power, etc.). As part of these regulations it is usually forbidden to interfere with the reception of radio waves.
So if what you do causes your neighbours to have reception problems, then yes, that will most likely be illegal.
In France, the government agency responsible for these problems is ARCEP (Autorité de Régulation des Communications Électroniques et des Postes). If someone notices reception problems, they can complain to ARCEP, as explained for example on the page Le traitement des plaintes en brouillage ("Handling of complaints about jamming").
While you will probably not go to prision for jamming reception, you could have to pay a significant fine. This article on cell phone jamming mentions a penalty of "up to six months in prison or a 30,000 € fine" for "selling or installing" a cell phone jammer. In addition to that, anyone harmed by the reception problems could sue you in civil court and try to collect financial damages (how much that would be will be up to a judge to decide).
That said, note that a faraday cage around your house should not hinder reception outside your house. A faraday cage only influences reception inside the cage, not outside. However, that is off-topic here :-).