€12.8K is a good price, but no-one here can guess whether you are getting value for money.
Here are some relevant questions that can affect the value for money
What level of insulation are you getting ?
What quality of staircase ?
Does this include decoration and all fittings ?
Do you have a personal recommendation for the builder ?
Who is going to do a structural survey and confirm that building regs have been obeyed and that the conversion is structurally sound ?
Are the building regulations wrt fire safety for a bedroom be met or is it just going to be designated as attic storage space ?