The usual reason for the 'total useful floor area' being reported smaller than the areas input is that blocks have been drawn using internal measurements without accounting for the fact that blocks should be drawn using external measurements. If you are uisng internal measurements then you should either add in the block wall thickness when drawing block floor plans or else use very thin block wall thickness and continue to use internal measurements. In the latter case the minimum block wall thickness allowed by DesignBuilder is 0.001m.

You can find out more on block wall thickness at:

http://www.designbuilder.co.uk/helpv3/Content/Wall_Thickness.htm

Note that if you have already drawn your blocks using internal dimensions and don't want to redraw the model you can change the block wall thickness to 0.001m retrospectively by going to block level and editing the block model options.