Documentation can be as simple as writing down everything that would be on a receipt (amount, date, how paid(cash/check #), why paid). If requested from the IRS, you will need to create this. If you are renting or using a portion of the house, you need to allocate the costs (paid by you and not paid by you) to that portion. If you are not paying rent/house mortgage, then you'll need to allocate the portion paid by them (if any), to your portion of the home. You can do this based on square footage. Then, evaluate in total if you are paying more than 50%. Depending on the costs of the home (if any) and everything you spend on other living expenses, it's certainly possible to pay more than 50%.
