Paying the sales tax is straightforward. On your CA state income tax form there is a line that asks if you bought anything in the past year and now owe CA sales/use tax. Answer yes, fill in the price(s) paid, include the tax along with any income tax due.
Property taxes are handled by a different part of the government, and are due only for ‘airplanes’, not ‘airplane parts’. As soon as I registered and received an N number I got a letter from the state. I responded that I did not yet have an airplane, as I did not have an airworthiness certificate yet. They accepted that, and left me alone for a year. Airplanes are taxed annually effective Jan 1, although the tax isn’t due until aug. E.g., if you think your AW inspection will be in Dec, postponing it until Jan 2 or later will save you one year of property taxes. Also if you later sell your airplane anytime between Jan and Aug you will still get, and owe, a property tax bill in aug. I sent the state a copy of my insurance, showing declared hull value. They seemed happy with that. I hear that some county assessors are upping assessed values based on the high prices used RVs have been selling for lately. So far they’ve left me alone. They will send you a form every year, asking if you’ve made any improvements, e.g., new avionics, paint, etc.
Bob, if you don't pay tax on "aircraft parts" (I guess you are quoting me?), then why would you pay tax on the kit early, or why would there be late penalty, as you said above? You are not wrong, but you don't understand the nuance of what I am saying. Let me explain.
Yes, first money takers, err tax collectors be it Fed or State or County have different departments, income, sales, property. The USE TAX or sometimes luxury tax folks are different, but they are all revenue.
My point is who says what you are taxed on, ie value of plane. Bob you indeed buy "Aircraft Parts", not an airplane. You (the manufacture) made an airplane from parts. However normally manufactures make something and sell it right away. The new owner buys and pays value or use tax. The businesses pay taxes as a business. KEY HERE TALK TO ACCOUNTANT, sometimes being the manufacture, you can waive sales tax and or defer it (to the future buyer). This varies by state. The tax will be collected when manufacture (you) sell the plane. However if this is not a business, then this may or may not be allowed. Also individuals at these money taker, err tax revenue departments in your state/county are individuals. One may intercept the tax law different from another. My one round years ago in Washington state went well. I have heard horror stories. However society has changed and Gov are grabbing money as fast as they spend it like water down the river or toilet, depending on perspective.
Example RV costs builder $60K in all the materials. The builder paid state/local tax on $30K of those parts. The builder (manufacture) pays 100% of the parts. The labor is taxed by payroll tax in a large company. The plane with $60K of parts is now worth $130K. Many states want the FULL VALUE of the aircraft as if you bought the EAB plane as if finished for you. You pay tax on your own labor.
Some states/counties that is their position. They are going on value of the finished plane not "aircraft parts" as Bob says. However other revenue departments will accept your stack of receipts for kit, engine, prop, panel, and say pay tax on that (minus any tax already paid). THIS IS WHERE KEEPING RECORDS OF TAXES YOU PAID AS YOU GO IS IMPORTANT. 
Youtube MotoGrip channel. Mike built or had a SLING built, fully loaded. He did a video on the taxes.
The IRS Came After My Plane For Taxes
https://youtu.be/g-wGBQn1j8A?si=SFuhNRQyM8n70RhY
Some states, I know OP is California, in general will go to Trade-Plane, Barnstormers, etc.
They will want to tax you on a comprisable RV-(). About "aircraft parts", SAVE your receipts. Any sales tax (state/local) on parts you already paid can reduce your tax liability by the amount of tax paid. In my state if I pay to have an engine overhauled, parts AND LABOR by engine shop are taxed. So my state has NO PROBLEM charging me tax on my own labor...
Bob my point my friend is YOU MAY BE PAYING TAX ON YOUR LABOR.
Some states are great. Use tax can be nil to small on your Airplane or Boat. It may just be a flat registration. It varies. Consult your accountant or call your states tax folks.
There is LLC or making the plane a business. You have to go through hoops to set that up. In my case I do transition training. So it gets complicated, but not so much you can't educate your self. However now you may be paying business taxes. As I said the first time MOVE... Ha ha.
In my case I will not be registering my plane in my current domicile. I am moving for more reasons than tax, which is 9.75%, On a 180HP, CS Prop, IFR RV-7, No I am not giving them $10K in taxes. Most of my parts were bought sans sales tax. The only saving grace is they accept my receipts on my current project which would lower the tax liability by 60% or more. Still a lot of money.
BOTTOM LINE ANYONE READING THIS... Start your tax strategy early.
If you think you are going to register it with Aunt Dorthy in another state to lower tax liability, come again. The states do check N numbers. If your plane is parked and operated out of an airport they can easily check that. If it is registered to you with address X and the plane has registration at address Y. They flag it. You will get the "Records show you own aircraft NXXXX and live at 100 Airman Way, Skyline XX. Please call this number to discuss your tax liability. IT HAS HAPPENED. In Seattle one of the Internet/MS moguls (Paul Allen I recall don't quote me) had all his luxury cars and jets registered in Portland Oregon, cheaper than Washington state. The cars were parked at his home in the garage and driveway, at his home on Lake Washington Seattle. The Jets were parked at his hanger at Boeing Field (KBFI), Seattle. So they went after him.
In fact I long ago when I lived in Washington state, I flew my 1958 Piper Apache several times a week to BFI, commute to work. I landed and State Troupers were on ramp recording N-Numbers. Mine was one. I got a letter. I had bought it a year earlier. Never paid tax on it. It turned out the bill of sale was written for less than what I paid. I never noticed. Still it was a little unexpected expense.
Every state is different and even by counties within the state. Do your homework, keep records and consider tax accountant. As knowable as Bob is do your own foot work. It may vary by your situation and location. If you BUY a flying RV that is straight forward. They will tax you for what you paid. If you are lucky and live in a state where planes are not taxed or nominal registration consider yourself lucky.
My state, city has public utility which is government, so yes poorly run and they just jacked prices up 12%. Also they just raised personal car/vehicle yearly "Axial" tax $25. I will stop there, but needless to say services of both city and utility are terrible. Time to move not only for tax but social issues.
