I have a capital gain taxes question:if I sell my home for 510k and provide the seller 10k seller credit towards escrow costs, would I be paying capital gains taxes in California for that additional 10k?

If you owe nothing or if you owe $1million it doesn’t affect the capital gain or the taxes due.  

It's like asking whether you can lose weight if you don't exercise.

The §1245 Silver Lining: Turning Tax "Pain" into Strategic Gain“’That'll be $500,000 in ordinary income.'

Sure I have doubts now that I’m midway through renovations…but I feel the contacts I’ve gained in a new city (electricians, plumbers, etc) are worth their weight in gold.

So the (presumably) two people currently on title would get $500k in capital gains excluded.

The insurance payout also impacts your taxable gain.

She is not married so her capital gains exclusion is only $250k. 

So it's weighted returns. 

I'd recommend looking at costs. your build cost is going to be well over other homes because of the fixed costs like slab, Hvac, roof, etc. check the market drivers and weight the benefits of short term versus long term. long term is no furniture and help pays mortgage but at this size it doesn't really cover everything. we didn't find a lot of economies of scale in single family detached. can you build more than one unit?