Just to round numbers , say you were to buy a house for 1 million put 500k capital improvements over last 15 years . Cost 1.5 mil.
Then sell it for 3 million - capital gains of 1.5 mil less the 500k married couples allowed to deduct on primary residences --- Taxable capital gains of 1 million
1 month later in same year you buy a home for 2 million .
How would that all play out at the end of the day for capital gains tax ?
Thank you

Just to round numbers , say you were to buy a house for 1 million put 500k capital improvements over last 15 years . Cost 1.5 mil.
Then sell it for 3 million - capital gains of 1.5 mil less the 500k married couples allowed to deduct on primary residences --- Taxable capital gains of 1 million
1 month later in same year you buy a home for 2 million .
How would that all play out at the end of the day for capital gains tax ?
Thank you

The correct term for the "Cost 1.5 mil." is "basis", not cost.

Capital gains rates vary from year-to-year, and I'm not familiar with any "500k married couples allowed to deduct on primary residences"; not to say it is inapplicable, just that I don't know about that.

I believe that the purchase of a second home for $2 million may (if taxpayers qualify) make the sale not a taxable event, so that no capital gains tax applies. If this is a hypothetical question, that's my off-the-cuff opinion, take it for what it's worth. If you are seriously contemplating doing this, consult with a tax professional.