Architects involvement is more common, from a very early stage in the project. You might also like to discuss your project with an architect at the pre-purchase stage, rather than after you have purchased the property. Although an architect is not usually involved until later on, when the plans need to be drawn up for the project (a strict requirement of planning permission for projects having more than 170 square metres of floor space) it is less common before the purchase.
But if you have questions about how feasible your planned project is it would be best to resolve them early on, even at the risk of payng an architect who tells you the project is not possible. Such queries might include:
– Is the existing building sufficient to support the weight of a new roof?
– Is the ground where you are hoping to build a large extension stable and secure enough to support the new building?
– Is your budget sufficient for the project that you are planning?
We have never personally used an architect at this stage, but that is partly because it never really occurred to us to ask whether the building was strong enough to support a new roof etc . We simply assumed, through our ignorance, that if there was an existing roof, then why not a new roof? That ignorance subsequently caused us some difficulties (not insurmountable), and we would probably behave differently with any future renovations.
In general, if you do have any doubts (or recognise that you should have doubts if you knew more about it) it must be preferable to spend a few hundred euros early on in the process, rather than wait until you have actually bought the property.
I would of course reommend that you use a local architect, who will have experience of the local techniques and conditions.