Readers' forum : Conversion liabilities

I act on behalf of a client who owns a commercial property that has been let to his trading company for about 20 years. The company then sublets part of the property to another trading company. There is no option to tax in place.

The client has obtained planning permission to extend the property to about twice the original size and convert the whole building into four flats. Each flat will consist of part of the existing building and part of the extension.

My first question is on VAT. Is it possible to consider the whole conversion as zero-rated because it is a ‘non-residential conversion’? If not what rates of VAT should apply here? If the client undertook the conversion without using a main contractor could say the trading company act as contractor? The plan would be for the company to claim input VAT on the expenses...