No. You must amend the prior return. It doesn't matter if this was personal or business, your return must be correct and complete for a given year, containing all income and adjustments that pertain to that year.
In this case, I think you probably mean that you didn't include some prior improvements in the cost basis of your home when you calculated the capital gains. If you need to adjust the cost basis, you have to amend the return that includes the sale.
If you did mean that you forgot to include capital improvements on commercial or rental property, you still have to report the sale correctly in the year it happened, but you may also need to account for depreciation and recapture, especially if the improvements happened in a prior year.
