Income boosters are liable for the mortgage, so if the owners stop paying, the booster's credit file could take a hit. In such cases, the booster may decide to contribute to the payments.

Outside of this, boosters don't have to contribute at all.

If they do contribute, we will treat all money paid toward the mortgage payments as a gift.

If you want to set up a declaration of trust or loan agreement to protect everyone's financial interests, you can do this by instructing a solicitor.