Short answer. By your answer — and only your answer. In expropriation, a defendant who objects to the taking shall serve his answer within the time stated in the summons, and that answer must adduce all his objections and defenses to the taking of his property, because defenses not alleged in it are waived.
What the law says
The answer shall specifically designate or identify the property in which he claims to have an interest, state the nature and extent of the interest claimed, and adduce all his objections and defenses to the taking of his property.
Rule 67, Section 3 — Defenses and objections. Read the full provision →
The answer carries everything
Expropriation procedure concentrates the landowner's entire opposition into one pleading. The answer must specifically designate or identify the property in which he claims to have an interest, state the nature and extent of the interest claimed, and adduce all his objections and defenses to the taking. Every argument you have — that the taking is not for the purpose claimed, that the expropriator lacks authority, that your property is not the one needed — belongs in that document. The rule adds a hard consequence: a defendant waives all defenses and objections not so alleged. There is a small mercy — the court may permit amendments to the answer to be made not later than ten days from the filing thereof — but the design is one shot, taken carefully.
What the answer cannot contain
The same provision strips the answer of the usual add-ons: no counterclaim, cross-claim or third-party complaint shall be alleged or allowed in the answer or any subsequent pleading. An expropriation case is not the forum to sue the government back or to drag in other parties; it decides whether the taking may proceed and what must be paid, nothing else. Claims you may have against the expropriator on other matters must go to a separate case. Knowing this in advance keeps you from wasting your answer — and your deadline — on material the court will disregard.
If you do not oppose the taking itself
The rule also provides for the owner whose real fight is about price, not principle: a defendant with no objection or defense to the action or the taking may simply file and serve a notice of appearance and a manifestation to that effect, identifying the property he claims an interest in — and thereafter, he shall be entitled to notice of all proceedings affecting the same. This keeps you inside the case without contesting it. Crucially, the compensation stage remains open to everyone: at the trial of the issue of just compensation, whether or not a defendant has previously appeared or answered, he may present evidence as to the amount of the compensation and share in the award.
Build the answer like the case depends on it
Because it does. Within the time stated in the summons, get counsel and assemble the record: your title and tax declarations establishing the nature and extent of the interest claimed, the expropriator's stated purpose against what is actually planned, and any ground for saying this property or this much of it is not properly taken. Plead every objection, however alternative — the waiver rule punishes the argument you saved for later. And even while contesting the taking, prepare your valuation evidence in parallel: appraisals and comparable sales, so that if the taking is upheld you arrive at the just-compensation stage ready to prove what the property is truly worth.