A mortgage is contingent on the income you report. If you're unable to report your income it cannot be considered. If you pay taxes on your income then you can list it as income with the mortgage company.
I suggest you talk with the company with whom you applied for a mortgage and told you that you could afford a mortgage of $X. They will tell you the basis for their decision and how you can improve the basis to qualify for a higher amount.
Do keep in mind that one of the reasons we as a country are in fiscal difficulty is because of all the sub prime mortgages that were approved. Those mortgages allowed people to purchase more expensive homes than they would ordinarily qualify for. You do not want to be one of those people who over time discovers that they cannot pay the mortgage.
When you plan a budget for buying a home you must include money for upkeep and repairs. Depending on the age of the home you buy that can be considerable. You will have expenses as a homeowner that you do not have as a renter and many of those expenses are not obvious.
When considering the amount that you pay for housing, you need to not only include the mortgage payment but also the cost of insurance, taxes, upkeep and repair. The figure I remember hearing is 30% or 1/3 of your income after state and federal taxes.
After you SWH: Since your income is "under the table" it cannot be considered when determining the amount you can afford for a mortgage. If it were to be considered, the IRS would get involved and you'd owe back and current and future taxes on that income. You just cannot have "it" both ways. It sounds like your income is considerable if you think it would help you get a higher mortgage. My value system would not allow me to not pay taxes on it. Sounds like you don't realize you are breaking the law.
Hmmm. Please don't blame the other person. It's your decision based on your values as to whether or not you pay taxes. My comment is based on your wish to have it both ways. One to not pay taxes and two to still be able to claim your income. I might decide to not report income but I would recognize that it's not income that would count towards anything. I would recognize the consequences of my choices.
I suggest that, depending on the amount of your income, you are doing what helps the other person at the expense of what would help you. You can now file an amended tax return and claim your income if it's sufficient enough to increase your loan amount. The only person who can tell you if that's the case is the mortgage company.