Of course he paid for nothing, someone gave him the money.

According to the Uniform Commercial Code, a structural addition can be treated as a "fixture".

A structural addition can be treated as a fixture under the Uniform Commercial Code (UCC) if it becomes so related to real property that an interest in it arises under real estate law, but the determination ultimately depends on common law tests for fixtures. The UCC governs the priority between security interests in such items that become fixtures, but it leaves the definition of a fixture itself to state law and established common law tests. Key factors in this determination include the manner of attachment, the purpose or adaptability of the item to the real estate, and the intention of the parties to make it a permanent part of the property.

The "Fixture" Definition:
Under UCC Section 9-102(a)(41), goods become fixtures "when they become so related to particular real estate that an interest in them arises under real estate law".

Priority Rules:
UCC Article 9 (specifically Section 9-334) provides rules for determining the priority of security interests between competing chattel (personal property) security interests and real estate interests over items that have become fixtures.

How to determine if a structural addition is a fixture
Since the UCC defers the definition of a fixture to real estate law, courts use common law tests to decide. These tests generally look at:
1. Manner of Attachment:
How is the item attached? Is it nailed, screwed, glued, or integrated into the structure? Removal without significant damage suggests it is not a fixture.
2. Adaptation or Purpose:
Was the item specially designed for the property, or is it an integral part of the building's function or design? For example, a custom built-in bookcase is more likely a fixture than a freestanding one.
3. Intent of the Parties:
This is often the most important factor. The intent of the party who annexed the item to make it a permanent part of the real estate is critical. This intent can be shown through overt agreements or the overall relationship between the parties.
Why it matters
Security Interests:

A structural addition that is treated as a fixture falls under real estate law for priority purposes, while the original goods remain personal property under the UCC.

Financing and Collateral:

Understanding whether an item is a fixture is crucial for financing and ensuring that a security interest is properly perfected in both the fixtures and the real property itself. A lender financing an addition will need to record their security interest in the county land records to secure their interest in the fixture.