Back of fag packet calcs suggests to me that this is might cost say �8k. But it could be +/- 30% of this.
As always in this situation you assure competitive environment by asking 3 good builders to quote. Don't say you are asking for 'lots of quotes' - 3 means each builder has an average 33% chance of getting the work. But do say you are intending to get three quotes - it tells them the work will be won competitively.
Price variability in your quotes will occur if there is asbestos in the existing roof to remove, or if they can't use the rubble of the existing prefab in the base of the new garage. Ask your chosen builders about this - makes it seem you're a girlie who knows what she's talking about.
Are you aware that some aspects of Building Regs (not Planning - you've sorted that) may apply to this structure because it is greater than 30 square metres in area? This is likely to be an issue if any aprt of your new garage is within 5m of your house, I think. It means that Building Control will want to inspect the foundations.