It is entirely possible to obtain a mortgage for a new build property however, the bank will not lend against a property unless it has a habitation certificate and is entered (or the entry is simultaneously being made) on the land registry. This means that the mortgage can only be granted once the property has been built and therefore stage payments made during the build process can not be financed by Mortgage.

In any event the property is not the buyer name until completion and so there is nothing for the buyer to secure the mortgage against. It is possible to obtain pre-approval for a mortgage on a property that is not yet built and to fix the mortgage offer until the property is completed. 

There used to be a common practice of securing an LTV that was higher than the final payment due to the builder and using the excess to cover purchase costs such as IVA and notary fees however this is no longer possible, the maximum mortgage granted can be no more than the final payment due to the builder. Alternative forms of finance may be available, for example releasing equity from another property or arranging bridging loans however these forms of finance can be costly and should not be considered without professional financial advice. I

Even if you have the funds in place to cover the stage payments and purchase costs and only need a mortgage to cover, for example, a final 50% payment to the builder it is potentially risky to commit funds to the a purchase, well in advance of completion knowing that circumstances could change making a mortgage unobtainable when completion is due. This risk can be mitigated by obtaining a specific new build offer in principle which pre-approves the borrower for the mortgage of a specific new build property for up to six months.

In order for the bank to commit to the client the client must also commit to taking the mortgage when the property is complete and so the borrower may be expected to pay fees at the point of approval and pay for a valuation of the hypothetically complete property, in advance of the actual completion. 

Alternatively, if the property is nearing completion you may be able to negotiate a lower stage payment for the property itself, allowing as much of the value as possible to be mortgaged.