What exactly are you basing this on? The term "purchase monies" doesn't appear once in the Stamp Duties legislation. The legislation refers only to the "consideration" being paid in respect of the land, the dwelling house or the apartment in question. Consideration has a very specific legal definition. Stamp duty is a revenue raised by means of stamps attached to written instruments, eg a lease or conveyance, based on the value or consideration for the instrument. It strikes at the time the instrument is executed. It would therefore be struck at a time when my boyfriend is neither a party to the transaction/instrument nor contrubuting to the mortgage. The fact that he would subsequently come onto the mortgage after the event and start contributing to the "purchase monies", as you put it, is neither here nor there from a stamping point of view.