For 2024, I would designate your current home as your second home and the home under construction as your main home. You are allowed to treat the home under construction as your main home for up to 24 months prior to completion. Then, because this home will be your main home for all 12 months, you can use the monthly statement balance average method for mortgages B and C over all 12 months in 2024, inserting $0 in the months that either mortgage was not held. For mortgage A, you will have to use the beginning and ending balance average method or the statement balance average method over the 5 months it will be held in 2024.
With the numbers you provided, this will result in a total average balance of $796,583 and a deductible interest percentage of $750,000 / $796,583 = 94.2%.
| Month | 2nd Home | Main Home |
| | Loan A | Loan B | Loan C |
| Jan | 335,000 | 139,000 | 0 |
| Feb | 335,000 | 139,000 | 0 |
| Mar | 335,000 | 289,000 | 0 |
| Apr | 335,000 | 464,000 | 0 |
| May | 335,000 | 544,000 | 0 |
| Jun | | 624,000 | 0 |
| Jul | | 704,000 | 0 |
| Aug | | 704,000 | 0 |
| Sep | | 783,000 | 0 |
| Oct | | 0 | 383,000 |
| Nov | | 0 | 383,000 |
| Dec | | 0 | 383,000 |
| | Average | Average | Average |
| | 335,000 | 365,833 | 95,750 |
