Hide it in plain sight.

You seem to have reasonable carte blanche when it comes to the secret base. One problem with the ideas so far is that you could be tracked down by your use of electricity, water or heat. So instead you build an appartment complex with several tenants. You own an appartment on the bottom (or if its luxurious enough you own the entire bottom). Beneath the utilities basement you have another basement where you will build the base of operations. Perhaps make it some off-the-books work with the contractor so it doesn't show up on the building plans.

You make sure the ventilation system of the building is connected to the lowest basement. You then do some work so the electrics and water of the basement are drawn from the appartment you own. This way any electric and water usage will not be unnoticed.

Make the bottom basement a gym for a short time (or have illegal activities take place there, out of your knowledge ofcourse). Eventually the gym will be shut down for monetary or legal reasons and you brick it up completely.

Now you have a basement that is there, people even know its there, but its no longer accessible and in use, right? There's hundreds of small sections no longer in use all over the city, doors and rooms that some people know technically exist but you walk passed it because there's nothing there. Its ventilated, it has running water and electricity that are billed to an adress and since its an appartment complex no one can wonder why there is random heat or electricity at that point, especially since its right beneath the utilities basement where you likely have the central water heater and electric cirquits for the building. And hey you live there, so you can also sit back and relax whenever you feel like it.

Hide it in plain sight.

You seem to have reasonable carte blanche when it comes to the secret base. One problem with the ideas so far is that you could be tracked down by your use of electricity, water or heat. So instead you build an apartment complex with several tenants. You own an appartment on the bottom (or if it's luxurious enough you own the entire bottom). Beneath the utilities basement you have another basement where you will build the base of operations. Perhaps make it some off-the-books work with the contractor so it doesn't show up on the building plans.

You make sure the ventilation system of the building is connected to the lowest basement. You then do some work so the electrics and water of the basement are drawn from the apartment you own. This way any electric and water usage will not be unnoticed.

Make the bottom basement a gym for a short time (or have illegal activities take place there, out of your knowledge of course). Eventually the gym will be shut down for monetary or legal reasons and you brick it up completely.

Now you have a basement that is there, people even know it's there, but it's no longer accessible and in use, right? There's hundreds of small sections no longer in use all over the city, doors and rooms that some people know technically exist but you walk past it because there's nothing there. It's ventilated, it has running water and electricity that are billed to an address, and since it's an apartment complex no one can wonder why there is random heat or electricity at that point, especially since it's right beneath the utilities basement where you likely have the central water heater and electric circuits for the building. And hey you live there, so you can also sit back and relax whenever you feel like it.