Yes, you will need plans for the planning permission and building regs drawings. Does the £1500 (don't forget the vat) include making the application? Sounds about right. £1100 a square foot sounds a litle high - usually more like £1000, but that depends on what you're having done.
Really depends what you are having done my hourly rate is £67 so that equates to almost 3wks of my time. Which if it's easy to moderate sounds fine Remember he needs to measure up and draw up the existing plans and elevations then do the proposed ones. Planning applications often require a fair bit of supporting information but it depends on your local authority. He also needs to provide information and details for building regulations. Please check he's an architect who carries proper insurance and registration tho and not a plan drawer, architectural consultant/designer etc as this gives you greater protection if things go wrong and definitely means they are qualified to do what you need rather than just own a pc and a CAD package
Agree with wonky legs to check they are registered architects.
We hired one for a loft conversion, it cost us £1200 for plans to submit to council. It didn't include any of the application work, fee or building regs. The plans kept coming back not what we wanted and after some research I found out they were architectural designers as opposed to trained architects