Hi Miranda,
The RIAI set out a suggested fee of the contract sum of between 9 and 11 percent, but you may find other (fixed) fee scales if you invite a registered Architect for a fee proposal. Also, what will bear on your fee cost will be how complex the considerations are for the given works, e.g. will be where the house is located (e.g. are you in one of the Dublin county councils, or Wicklow, or another local authority), how old the house and area may be, if it's a conservation area, and other factors (e.g. with first floor extensions for example with South Dublin co co there is a specific mention in their guide to assess overshadowing of neighbouring properties.
Brian
BM ArchitecturalServices