It is very difficult to comment specifically about this case without sight of the actual policy chosen. However, in general terms it is safe to say that the cost of reinstating an historic property is likely to be much greater than that of a standard or modern home. Therefore it is very important that the sums insured are maintained at appropriate levels. Ensuring that all the historical features are reinstated in the event of a claim will add to the cost, thus an uplift in the sums insured may be required to allow for the work of specialist tradesmen as well as the old forms of construction that were used to build the house.
In particular, listed buildings will usually take longer to rebuild as you will need to keep the planning department aware of the progress. The amount of cover that is needed for the provision of alternative accommodation costs whilst your home is restored to its former glory will also be an important issue. With a modern common estate type house rebuild costs may be closer to the purchase price because in comparison they are far simpler to both demolish and rebuild. Their overall construction, for a proportionate floor area, will probably not involve the same amount of materials and these will usually all be readily available, mass produced and therefore cost less.