If your business is on a cash basis, you report income and expenses in the year they are incurred.
On your 2020 return, report the expenses you paid (contractors, supplies, etc.) on the Schedule C for your business. You would also report any income you received during the year (2020).
On your 2021 return report the sale of the spec home. To do this you would have to calculate the basis of the property. Briefly, your basis is what you paid for the land, plus improvements (since this is new construction the entire house is the improvement), plus some of the closing costs when purchased.
Please these resources for additional information on calculating basis:
