Cost of an extra garage

Hi all,
We just got a revised tender from the builder.
as our frontage is over 18m, we asked them to add an extra garage,and convert one into a home office, the builder quoted for around 15K from the sales lady .
first tender was a bit surprise, it cost about $15K plus $2K window cost, but the office width is only 2m, we thought it was a typo, as the whole building width increased from 11.5m to 14.5m, then the increased part is the office , shall be 3m, so we asked them to change it
guess what? when the revised tender came, it became $20K plus window cost.
can anybody tell me how much does an extra garage actually cost? does this sounds reasonable?

Thanks in advance for your opinion!

It probably depends on the structure; if the new design means they need extra steel in the structure to span further, then it will add a disproportionate amount extra. Similarly if it's a two-storey design & an upstairs wall sits over the garage wall in the original design, and the changed design moves that downstairs garage wall, then extra internal structure & possibly steel is needed.

Just as an example, to convert single carport to a single garage cost me about $7,000 even though the slab and roof were there originally. So I am not surprised that the quote is for $15,000 if you need to add extra slab and frame and roof etc.

Thank you for your replies.
I will go check the plans to see the difference, actually, we do accept the 15K charge.
my point is: when the builder drafted 1st plan, they expanded the width from 11.5m to 14.5m (I can measure from the draft) on their proposal, that means 3m's width for my home office. but the office actually was 2m on their tender, where does this extra 1m go? can they argue that draft is not accountable? now they want to charge me for another 5k .

Do you have a plan for the "2m office" design, and an original plan, that you can post here?

There are Eagle Eyes around here that might spot where the space went.

Ask them to directly cost it out as it seems high. I am in Perth an a fully rendered inside and out, freestanding garage 9m x 7m with boundary wall along the long side and tiled roof was 45k. The height is also at 32c.

i asked my builder (in perth) about the cost for the third garage and it was $10,000 to $15,00 with roller door.

we didnt go with it.

we did how ever go with adding a 1m through the house making the study, larder and theatre bigger. that was about $6,000

Thanks for all your replies.they are very helpful.
As we only paid $1000 for survey and tender, the plan they gave to us is just a very basic site analysis & site plan.
we are going to see them in the weekend to ask details.

We raised our question with the sales.The sales checked the draft drawings, admitted the error, called the estimator, got the reply was the error in the draft drawings. so, 15K for a 2m wide garage converted office! If we want 3m office, it would be 20K!!
when we asked the garage cost, the sales pull out the figure, for our design, the price is 13k, then need to consider carpet and insulation, and extra height of the ceiling.
we totally lost, how did the work done?