If I get it, how long will it take to get the land deeds changed for the site ?
Is there is a fixed wait time?
To transfer a site into your name, if a solicitor is involved, they will need the title deeds, a map ( if the site is part of a larger plot, this map will need to have the area in transfer certified by an engineer) and your personal details- name, address, RSI numbers etc. Once the solicitor has these she or he can draw up the documents of transfer fairly quickly - if there is a rush, within a week. You will then all have to sign ( independant legal advice may be needed for your parents depending on their wishes and the circumstances) and once the docs are signed you are the beneficial owner and can mortgage it and build immediately.
Can I build before the land ownership is changed.
Yes, but I wouldnt recommend it for various legal reasons ( the most simple of which is that you DON'T own it) and tax issues.
Do I have to go to a solicitor or can i go to the land registry office directly?
You don't have to go to a solicitor, but do you know how to transfer ownership? Do you know what steps to follow, what documents will be needed- not just now but in the future for any bank that you want to get a mortgage from or if you want to sell. I would recommend going to a solicitor.
Depends on the title, whether it is part of a folio or a full folio, depends on the value of the site, whether you are getting a mortgage etc- I would recommend you ring for eg your family solicitor or make an appt to see them and ask them for a quote( you won't be charged for getting a quote).
Will my partner have to pay tax on the site and how is it calculated?
Depends on the value of the site- I assume you NEED to put the site into joint names, rather than into your sole name?