A builder would be better to give you an idea of costing/design as designer usually design something that is out of your budget as they don't necessarily know the true costs of renovations. We see it all the time, client come in with plans from drasftpeople/architects and they are never within their budget but everyone happily parts with money to pay the designer but are not happy to pay a builder for a design which will more likely be within your budget and your money more wisely spent than having paid $1000's of dollars for a plan that gets thrown in the bin. Try a design/construct builder in your area as they can work with you on the design and costings

Just to stick up for building designers & architects quickly, like every profession there are a mixture of good and not so good. There is also no reason you can't have both designer/builder involved throughout the whole process (or a design/construct builder as suggested, which is a similar situation).

There are always those who have a limited understanding of cost/construction, but most reputable building designers/architects will take budget very seriously. Generally we work on a "per square metre rate" in the very early design stages which often comes surprisingly close to the final build cost, well before we've worked through any structural details. As mentioned though, renovations do have their own hidden costs, especially 'going up' (assuming you mean adding another storey)

To answer your question directly though, my recommendation would be to first talk to an architect, a building designer or your builder (who may direct you to one of the former). An interior designer may not have the structural knowledge required for this job, and certainly won't be registered, a requirement in order to get your building permit. A registered designer will make your ideas feasible, help you make the most efficient use of your space, get it through planning if required, and will keep it all within the building regulations. Even if you do have a builder in mind, it can still be worth having sketch plans done first, to give them a much clearer understanding of how to help you.

Good Luck,

Matt

The first question I ask a client is: What is your budget for the construction?

Many are quite surprised that I bring up money before asking anything else, even for a basic deck addition or a new dwelling the question must be asked.

I too initially work to a ball park 'per square metre rate'. This get's sharpened up as the design progresses. I also check out potential 'land mines' like electrical and plumbing that needs maintenance and/or upgrading. Is the existing site and structure suitable for the additions, etc. I highlight these to the client as these could add thousands to the budget.

If your chosen designer be it architect, building designer or a good draftsman doesn't ask the budget find another one!!

Don't just choose an architect because you want good design, choose a good designer, they could be a draftsman that design's exceptionally well, a building designer with solid experience or an architect that designs well. Design experience is the key to any project as this affects budgets, efficiency, liveability, etc..