It just happens that I am researching the right of rescission myself today. Here is what I found in answer to your question:
Commentary 226.23(a)3
When a consumer buys or builds a new dwelling that will become the consumer's principal dwelling within one year or upon completion of construction, the new dwelling is considered the principal dwelling if it secures the acquisition or construction loan.
However, it goes on to stay in 226.23(a)4
Special rule for principal dwelling. Not withstanding the general rule that consumers may have only one principal dwelling, when the consumer is acquiring or constructing a new principal dwelling, any loan subject to Regulation Z and secured by the equity in the consumer's current principal dwelling (for example, a bridge loan) is subject to the right of recsission regardless of the purpose of that loan. For example, if a consumer whose principal dwelling is currently A builds B to be occupied by the consumer upon the completion of construction, a consutruction loan to finance B and secured by A is subject to the right of rescission. A loan secured by both A and B is, Likewise, rescindable.
My conclusion: Once you take home A as collateral it is rescindable!
Hope that helps!