If $0\mapsto A \mapsto B \mapsto C\mapsto 0$$0\to A \to B \to C\to 0$ is a short exact sequence of groups and $A$, $C$ are of type $FP_{n}$, then so is $B$ (https://arxiv.org/pdf/1611.03759.pdf Prop. Reference: Proposition 2.2 in Homological finiteness properties of fibre products by Kochloukova and Ferreira Lima (arXiv link).