Improvement without contract taxable when sold within four years after completion--Exceptions--Determination of gross receipts

If any person, except for a person making an isolated or occasional realty improvement and who does not hold himself out as engaging in the business of making realty improvements, makes a realty improvement without a realty improvement contract and subsequently sells the property containing the realty improvement or the realty improvement at any time subsequent to the commencement of the project and within four years of its completion, the gross receipts from the sale of the realty improvement shall be subject to the excise tax imposed on the gross receipts of contractors engaged in realty improvement contracts. In determining the gross receipts from the sale of such realty improvements when land or land and improvements completed four years prior to sale are included in the sale, land and such improvements shall be valued at cost or fair market value, whichever is greater, and the portion of the gross receipts attributable to land or land and improvements completed four years prior to sale shall be deducted from the sale price.