In my experience, architects don't have a clue about costs. They're designers who will advise you on how to make best use of space. But you then need a quantity surveyor to cost it. Technically you might even need an engineer to advise on where structural works might be required.
A builder will tell you how much a standard extension might cost. But bi-fold doors, a roof light, marble top kitchen, and garden landscaping easily adds 150k to your budget.
Unless you know someone you really trust, it's difficult to get advice on the full picture.