There are nuanced tax rules regarding income derived from renting your building and/or parking lot, depending on its relation to your core purpose.
The following is from the IRS Tax Guide for Churches & Religious Organizations:
Rental Income vs. Other Church Income
Generally, income derived from the rental of real property and incidental personal property is excluded from unrelated business income. However, there are certain situations in which rental income may be unrelated business taxable income:
if a church rents out property on which there is debt outstanding (for example, a mortgage note), the rental income may constitute unrelated debt-financed income subject to UBIT. (However, if a church or convention or association of churches acquires debt-financed land and intends to use it for exempt purposes within 15 years of the time of acquisition, then income from the rental of the land may not constitute unrelated business income.)
if personal services are rendered in connection with the rental, then the income may be unrelated business taxable income.
If a church owns a parking lot that is used by church members and visitors while attending church services, any parking fee paid to the church would not be subject to UBIT. However, if a church operates a parking lot that is used by members of the general public, parking fees would be taxable, as this activity would not be substantially related to the church’s exempt purpose, and parking fees are not treated as rent from real property. If the church enters into a lease with a third party who operates the church’s parking lot and pays rent to the church, these payments would not be subject to tax, as they would constitute rent from real property. Whether an income-producing activity is an unrelated trade or business activity depends on all the facts and circumstances.