I'm married, we're still both alive, and hope to stay that way.
Around 1985 I used an inheritance to purchase property that was our principle residence ever since. Title was in my name.
In 2017 we started planning to move. In order to obtain a suitable bridging loan, our lawyer had me add my spouse's name to the property title being used for collateral so that her income could be taken into account. No money changed hands.
In 2019 we sold our current house.
I assume the ACB in 1985 was the price of the land plus the cost to build, which I can estimate.
Can I fill out the principle residence worksheet just as myself - enter the ACB, sale price, etc. - or do I need to invent a proportion of ownership with my spouse, change my 2017 return to reflect a deemed disposition, and have her fill a 2019 PRWS giving the imaginary ACB from 2017 ?
If you gave part ownership to your wife in 2017, then that would be a "deemed disposition" for you. You should amend your 2017 return to reflect that disposition. For 2019, since both you and your wife have ownership of the home, both of you would fill out the principle residence worksheet on your respective returns.
The Adjusted Cost Base is usually the cost of a property plus any expenses to acquire it, such as commissions and legal fees. It can also include capital expenditures, such as the cost of additions and improvements to the property. For a deemed disposition, the ACB would be the fair market value of the property at the time of acquisition.