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1. Am not entirely sure about this as this concept of "changing from ordinary laon to staff loan" would best be answered by the mortgage consultant advising you in this matter as I am not aware of the mechanism of it. If it is the same as refinancing, then you're looking at legal fees plus stamp duty as well. The legal fees will be scaled according to the loan sum.
2. You can't add your name to the SNP. Ownership depends on the title, not the SNP.
Two scenarios:
a) If there property is with title, then your gf can transfer half share in the property to you. This is called subject to charge - and this can only be done with the consent of the bank. If they do not allow same, you have to refinance it. Fees and costs for refinancing will be the same as above.
b) If the property is without title, the only document to show beneficial interest is the deed of assignment (DOA). As she is the sole person in the SNP, the property is wholly assigned to her (assuming it is a subsale as you did not mention). She will then assign it to the bank in exchange for a loan. At this stage, there is no change of name that can be done, as she is not the registered proprietor of the property, but is merely the beneficial owner. To include you as a person who has a right by way of assignment, she would have to refinance it, assign half of her beneficial right over the property, and both of you would be the borrowers for the new loan. Fees and costs for refinancing will be the same as above.
With regard to legal fees for transfer - as the subsequent transfer will be for love and affection, the SRO provides that the legal fees for transfer shall be a reasonable sum, but in any event shall not exceed the scaled fees in accordance to the market value of the property.
For option 2, you would need another document to secure your interest in the half-share, and that is a deed of trust. This will confirm that your gf is holding half-share of the property in trust on your favour. Your lawyer would have to prepare this document to secure your interest, as there is nothing on paper [looking at the SNP and the loan docs] to suggest that you have a right over the property.
Option 1 is more viable and is less complicated than Option 2.
Good luck

That is a very informative answer. I will study again the pros/cons and also the cost involved.