Google sheets: Case statement or ifelse formula to modify a cell's value based on the value of another cell?

1. Based on a validation, if A1 = 20, then B1 = 5.
2. Based on a formula, B2, B3, B4, B5, B6, and B7 = B1.

Except, I want B2:B6 to equal B1 + D1, if C1 = True.

My question is: How do I make it so that the formula for one cell (B2) equals the sum of two cells (B1,D1) in the event that a third cell (C1) has a True value? If it is False, I want it (B2) to just equal the sum of one cell (B1).