After the cement is poured there are still a lot of things to consider.
First thing would be the design, so you should have a look here
Then you need to consider the different steps and cost to build an attached garage
as there might be some steps you could do yourself.
Also make sure you use the right material and reliable contractors.
Allow 15 to 20% extra budget as something costly may turn up.
Finally interior finish
is another important step, it depends what you are planning to do in there.Hope this helps.
Let us know how it is going.