You may wish to try http://architect-yourhome.com/
They allow you to pick the level of service you require.
The biggest influence in the price of the any quotation will be ‘time required’.
If you know exactly what you want and therefore the designer only needs to draw up one scheme then this is a lot cheaper than having to produce lots of varied options for you to choose from.
For example, I have compiled from scratch a design and Planning Application for £200 for one Client who knew exactly what they wanted. Another Client who wanted ‘just an extension’ ended up with a bill of £2,000 to arrive at the design they wanted.
Structurral Calcs and Design should be completed by a Structural Engineer (for a loft conversion).