self-build mortgage
> We've been told by a broker that we might be entitled to a preferential interest rate as we own our site and the full value of the completed house will be 60% more than what we are borrowing.
Not sure if ownership of the site counts specifically but many lenders offer preferential rates to borrowers who are looking for a lower than normal loan to value ratio (the amount of the loan required expressed as a percentage of the house value). See the mortgage best buys and list of trackers for example:
www.askaboutmoney.com/clu...#MORTGAGES
> We have never heard of this and need to know what the advantages and disadvantages of this might be.
Nothing devious - if you borrow a low LTV (e.g. < 40% or < 60%) then some lenders offer preferantial rates. That's all.
> Also our broker offers legal services for conveyancing, however we are unsure as to what legal services are required for a self-build mortgage when we already own the site.
I think that there are still legal/convevancing issues to be dealt with even if you own the site so you will probably still need a solicitor.
What sort of "broker" is this? Are they an Authorised Advisor, a multi-agency intermediary or a tied agent? And what are they claiming to be able to sort out for you? I would have thought that if they were any good then they could answer these questions directly? Have you considered shopping around for the required services and products (mortgage, mortgage protection life assurance, house insurance, conveyancing solicitor etc.) yourself?