Most likely, you report it as a capital gain on form 8949 and Schedule D. Alternatively, you report it as a business profit on Schedule C.
For more info, in making that determination, see a similar question at: https://ttlc.intuit.com/questions/3399983-tax-issues-regarding-flipping-of-a-house . The thread is about flipping houses, but would be applicable to building houses too.
And references at:
https://www.hrblock.com/tax-center/income/real-estate/flipping-houses-taxes/
https://fitsmallbusiness.com/taxes-on-flipping-houses/ https://www.lendinghome.com/blog/how-to-maximize-house-flipping-tax-benefits/