Short answer. Yes. If you incorporated your accessory into another person's principal thing in bad faith — without permission and knowing you had no right to do so — the Civil Code says you lose the incorporated item and must also pay the principal owner for any damages caused.
What the law says
Whenever the owner of the accessory thing has made the incorporation in bad faith, he shall lose the thing incorporated and shall have the obligation to indemnify the owner of the principal thing for the damages he may have suffered.
Civil Code, Article 470 — Bad Faith in Adjunction. Read the full provision →
Bad faith on the accessory owner's side
Article 470 of the Civil Code covers adjunction — the joining of a lesser (accessory) thing to a more important (principal) thing so that separation is difficult. When the owner of the accessory makes the incorporation in bad faith — knowingly and without the principal owner's permission — the Civil Code imposes a stiff penalty: he shall lose the thing incorporated and shall have the obligation to indemnify the owner of the principal thing for the damages he may have suffered. You lose the item and you owe compensation. There is no middle ground. The rule is designed to deter unauthorized unions of property.
When the principal owner acted in bad faith instead
The rule flips when it is the principal owner who acted in bad faith — for example, accepting or facilitating the incorporation while knowing it was unauthorized and then refusing to recognize the accessory owner's right. In that case, the accessory owner may choose between two remedies: demand that the principal owner pay the value of the incorporated item, or demand that the item be separated from the principal thing — even if that requires destroying the principal thing. Either way, damages must also be paid. The Civil Code treats bad faith by the principal owner as the more serious wrong and gives the aggrieved accessory owner the power of choice.
When one party acted with the other's knowledge and without objection
Not every unauthorized incorporation is bad faith. Article 470 provides that if either owner made the incorporation with the knowledge and without the objection of the other, both are treated as having acted in good faith. This matters because the good-faith rules on adjunction are considerably more forgiving: the principal owner typically acquires the accessory but must indemnify the accessory owner, and separation may only be demanded if it does not injure either thing. If you incorporated your item openly, the other owner watched and said nothing, and only later objected, that silence may be treated as acquiescence.
What this means practically
Before joining any material, part, or component to property you do not own — even if you believe it will improve the property or that the owner will eventually agree — get written consent. Bad faith in this context is not just malicious intent; it includes acting in the face of a clear lack of authority. If a dispute has already arisen and you incorporated something without permission, the key question is whether the principal owner had knowledge of the incorporation and an opportunity to object, and whether they did object. The answer to that question will determine whether the harsh bad-faith rule applies or whether a more balanced good-faith analysis governs.