I'm confused as to the situation - However, the MV is the value of the property as it physically stands with the extension based on comparable sales evidence.

Therefore to find out what it would be worth without the extension you would need to find comparables sales evidence of similar houses without extensions.

If such comparable evidence cannot be found then you would have to guesstimate the added value to the property by having the benefit of the extension. This is likely to be more than the cost of build but not necessarily and will be largely determined by the locality and what the trend is for that locality.

My answer probably makes no more sense to you then your question did to me...so we'll call it quits.