What defines a Giga factory? 
(From a Materials Handling perspective)

I suppose anything over 300,000 square meters if you are Tesla; the Northumberland factory is to be significantly bigger again on a 950,000 square meter site.

One of the key briefs in engineering these days is the ability to expand, retract, reorganise, extend, and above all build quickly. To do that you need to become practiced and that is the problem because practice doesn’t like innovation, since that implies change and change implies delay.  It is nearly always cheaper to build up rather than out, so multi-levels in manufacturing and headroom in warehousing, that way the Giga factory on 900,000 square feet instantly triples in floor space and apart from the change of level issues, one achieves more storage whilst the other more production.

At a guess I would say we are talking around 100,000 tones of steel and although they seem to be able to complete Giga factories in 7 months in the USA from first phone contact I think that would be overly optimistic here in the UK.

Equipment is bound to include automation, conveying, handling and storage, not to mention barriers to protect the workforce. Then there are offices, heating systems, fire prevention systems, and of course security. Loading bays and traffic all need to be carefully planned.