Which factor could potentially invalidate a real estate contract in Illinois?

In Illinois, a real estate contract can be deemed invalid if there is an improper signature on the contract. This is because all parties involved in a contract must provide their signatures to indicate their agreement to the terms outlined. If a signature is missing, forged, or otherwise improperly executed, the validity of the contract is compromised. A contract requires mutual consent, and this consent is typically demonstrated through signatures. Therefore, if the proper individuals do not sign the contract as required, it fails to meet the legal standards necessary for enforcement.

In contrast, the other options listed do not inherently lead to the invalidation of a contract. Lack of building code compliance may be a concern regarding the property but does not negate the existence or enforceability of the contract itself. Similarly, a property's age is not a legal factor that impacts a contract's validity. Lastly, a decrease in the home's value does not impact the validity of the contract; it may affect the market conditions, but the contract remains intact regardless of property value fluctuations after the agreement is made.