Tying a gable roof into an existing roof is a long expensive project usually undertaken only when there is a major addition or renovation to a house.
How to match a roof addition.
But it can be extremely difficult to match new materials to old.
It is not a job for beginners.
Also try to match finish materials such as siding roofing and trim.
Or if the addition is for a garage a lot of times the floor is lower than the house floor anyway and it can be adjusted up or down to make a regular framed wall come out slightly lower than the existing house wall.
You may need a structural engineer to help with design of trusses and rafters.
So if your house has casement windows for example oversized crown moldings or wide plank floors you ll likely want to repeat those design elements in the new addition.
If the ceiling of the new addition can be lowered an inch or 2 this gives us room to build a truss to match on the roof top.