Can I add remodeling cost before the sell to my old house cost base and replacement/improvement cost to my new house cost base

[x_m_z]

Recently I sold my primary house ( it was rented about 40% of the house while we were living inside for 10 years) in California and bought a new primary house in Washington state. I did remodeling before sell the old house (capital gain exceeded 500K ),  the new house I bought have many broken area which I have to replace and improve. Can I add these cost to the cost base for both old house and new house?

For the old house: remodeling cost before the sell was about 20K-30K, can I add these cost to the old house cost base?

For the new house: the furnace and central air conditioning are broken, roof and gutters are leaking, garage opening and gas cooktop are broken, . I have to replace these hardware with new and different model, can I add these cost to the cost base for my new house?

[Critter-3]

Yes, improvements are always added to the cost basis of the home. 

[Opus 17]

Be careful, the two transactions are entirely separate.  Any additions to the cost basis of the new home won't have any tax effect until you sell that home in the future.  Keep records as long as you own the home, but improvements to home #2 don't change the capital gains tax owed on the sale of home #1.

Also, not all the items you mentioned will be adjustments to cost basis.  You can include improvements, but not repairs.  Repairs maintain the property in as-is condition and can't be added to your basis.  Improvements (or betterments) make the property more valuable or extend the useful life of the property or its subsystems.  Improvements must also be permanently attached to the real property (the land, or anything permanently attached to the land).  Furnace repair is not a cost basis adjustment, but replacement would be (because it extends the useful life of the heating and cooling system).  Replacing a built-in cooktop would be an improvement because it is built-in, but costs for freestanding appliances are not counted as cost basis adjustments.  Patching the roof is a repair, but replacing the entire roof (or a substantial section) that gives it a new 15 year life cycle would be an improvement.  And so on. 

[Opus 17]

One more thing. And expense which would normally be in the nature of a repair can be included in the cost of an improvement if it is integral to the improvement. For example, painting your home to give it a “fresh look“ for sale is a repair and is not a permanent improvement to be included in the cost basis.  However, if you remodeled the kitchen, which includes painting after new drywall, that painting job is part of the cost of the improvement.