You should ask these questions of your local taxing authority. Many factors come into play and it depends on where you live.

Just about every tax assessor uses local rules.

Virginia has some state level guidance, but the localities (counties & cities) perform the actual assessments.

It is based on an average in an area, not a single value for a sale.

It is typically held slightly lagging (even though it is supposed to be 'market value') to avoid huge numbers of appeals.

Few would be wiling to sell for the assessed value, so appeals tend to not be very productive.

There is a fabulous nonprofit group here in Atlanta called the Fulton County Taxpayers Foundation. I found out about them by accident, and attended one of their classes last month. The class was taught by a former tax assessor, information they provided was very useful in regards to how taxes are formulated, how to appeal if you think your assessment is out of kilter, and how to gather your information for your appeal. Although their name is "Fulton County", the info they provided was valid statewide, and they welcomed anyone from any county in the state.

It was well worth two hours of a Saturday morning to attend, and the class was free.

See if you have a similar group nearby.

BTW, they had no political orientation or agenda that I could determine. They presented purely property tax related information.

Here is a link that might be useful: FCTF Fulton County Taxpayers Foundation

DIY'ers don't face property tax increases especially if they are smart enough to avoid pulling of permits only when absolutely necessary.

@mcgoverntheory: smart people make sure they get permits when they're legally required.

Smart people know ways to avoid paying higher taxes. Lemmings pay more.

Design your house for your needs and wants. Even if you try to outsmart the tax assessor, the next local administration and voters will change the parameters.

Only a lemming would be distracted by the shiny bauble of a short term tax break and ignore the long term cost of unpermitted work reducing potential resale value of his house. Not to mention, potentially endangering hidps family if he overestimates his DIY ability. But then, lemmings generally don't look ahead...

Tax breaks are not short-term. If they increase the value of your property, you will pay an ongoing higher rate till you sell or die. But then, lemmings generally don't look ahead...