Firstly, where is the car being rebuilt? If the work is being carried out by a restorer on his premises, then you should ensure that the restorer has adequate motor trade insurance in the event that your car or any parts wherefrom is lost or damaged while in his ‘custody or control’ to use the terminology. You should ensure that a written and pictorial record is maintained and kept off-site so that in the event of a loss, the insurers are aware of the condition of the car immediately prior to the event.
If the rebuild is being carried out in your own garage, then an ‘off road’ policy is likely to be suitable, particularly where you do not anticipate completion within the next year. The cover given includes accidental damage – to cover objects falling from shelves or storm damaged roofs, etc. the premium is nominally quite low as there is no road risks cover. If you foresee the car being back on the road within the next year, you may be better advised to start a low mileage policy, perhaps 1500 mpa. If nothing else, when the time comes, you will be able to tax and MOT the car, take it to the trim or paint shop, or whatever. The difference in premium is small, but the convenience of only doing the basic insurance paperwork once (apart from photographs and an up to date agreed value) could be appealing.
Incidentally, when a long term rebuild is involved and considerable tranches of cash are expended, do remember to keep the value up to date by submitting photographs of progress and written evidence of work carried out – invoices, etc. otherwise you will only get paid out the value agreed previously in the event of a claim.