It’s going to cost you a significant amount of money but hard to estimate without knowing sizes and what’s there. In CA, the demolition and fill in for a swimming pool requires a permit and inspections for the work. You will be required to drain the pool, cap all underground piping, especially gas, ensure that the shell is either demolished or has enough drainage holes in it to support the exiting ground water flow. All equipment will have to be removed and the structure will need to be filled and compacted. Some areas require specific fill material AND a certified compaction report by a geotechnical engineer. You will also have to amend/update the platt map for your property to show that the pool was removed.
It’s generally not worth removing an inground pool as it will impact the resale value of the property. A lot of people will be hesitant to buy a home knowing that there is something abandoned and buried in the backyard. And given the requirements in CA, disclosure is required and potentially very costly if you try to avoid it.
