sweat equity

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Work, especially manual labor, performed in return for a share in ownership, as of a home, or in the expectation of increasing the value of a property one has purchased.
• noun Sustained effort; hard work.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic, real estate, business An investment of labour, typically by the owner and often his or her family, usually in a small business or personal residence that increases the value of the business or residence.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun interest in a building that a tenant earns by contributing to its renovation or maintenance