When matched to the angle and proportions of other rooflines on the house your gable roof will add visual interest and character to the home making the addition look like an asset that belongs with the house.
How to match a roof addition.
You may need a structural engineer to help with design of trusses and rafters.
Rather than a single slope a gable roof consists of two slopes that meet at a peak running down the middle of the roof.
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.
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.
Also try to match finish materials such as siding roofing and trim.
It is not a job for beginners.
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.