how much to build a 3m x 5m office

[mpheating]

I just want a rough price what you will think it will cost to build an office 3m wide x 5m long x 2.3m heigh

............ _________________
............|.............................|
............|.............................|
.Garage |.............................Window here
............|.............................|
............|__window___door__|
Office will be tagged onto the back of a garage.

I want a 100mm cavity with block work inside

I have the following materials already:
1400 bricks, for the front and side where the window is
cavity wall full fill insulation
damp proof membranes
door and 2 windows

I need everything else which the builder is getting me a list so that I can supply as my best friend works at a merchants.

Builder has quoted me £5000 inc labour and rest of materials needed, and roof which will be flat felted.

footing are going to be roughly 600mm deep he said as he build the house extension 10 years ago for the previous owner.

external back wall is going to be block work same as inside

Do you think this is a good price because I havnt a clue

 

[mpheating]

can anybody help me?

 

[NS215]

best to get another couple of quotes from local builders then you can compare