Your cost basis in the property is what you paid for it.Period. Your own labor is not and can not be included either. Since your own personal labor can not be taxed, you can't use any value you would assign to your labor to increase the value. So you would add up all your receipts to get the cost basis, and possibly a few other things.
The cost basis of the property includes:
1) What you paid to purchase the land
2) What you paid for the structure you build on that land.
3) What you paid for any permits or licensing required in order to build or modify the land.
4) What you paid contractors, including labor costs that you paid to contractors.
Note the above may not be all inclusive.
Another option might be to use the taxable value from the local appraisal district when it was completed.
Actually, that's not only the worst option, but can only be used as the absolute last resort when there is no other possible way to determine the cost basis.
Remember, the property tax appraiser only appraises tax value - not market value. Typically, the tax value will be 30% or more below the market value or actual cost. the property tax appraiser generally does not go inside the property when assessing tax value.
Commonly, tax values are based on square footage of living space. In some locales the tax for what is referred to as "heated/cooled space" is different from other space (such as the garage which is not heated/cooled.) The open-air front porch may be taxed differently per square foot, than the heated/cooled space is taxed.
The best way to get the FMV of a property if you can't get the actual cost, is to have an appraisal done by a qualified, licensed and certified property appraiser. Of course, an appraisal 30 years after the fact really doesn't do the trick. But if the appraiser has their own records of similar properties going back that far, it's perfectly possible they can give you an accurate appraisal for the year it was built. However in reality, I've never known or even heard of a licensed property appraiser being in the business that long. So it's doubtful you'd find an appraiser with records back that far which they could reference to give an accurate value to a house built in the 1980's.
If it comes down to it (and I doubt it will) you can get a current property appraisal and compare it percentage-wise to the current tax apprasal. Then "do the math" based on the initial tax appraisal in the year it was built to come up with what "might" be a realistic number. But any additions or improvements to the property since it was originally built will have a big impact on all of that, thus making this endeavor fruitless.
