Additions should not look like additions.
I always try to respect the original architecture of the home when designing an addition, so that when the construction is complete, the new space looks like it has been there all along.
Unless your addition is a specialized type such as a solarium, always use the same exterior materials and accents on the addition as you have on the original home.
Alternatively, an ‘exterior makeover’ can apply a fresh theme to the entire home while the addition is being done.
Good planning can make wall-to- ceiling transition areas virtually seamless. Carefully specifying roof joist styles can avoid flat ceiling/vaulted ceiling awkwardness. ‘Rainscreen’ is a required construction detail for exterior finishes; designing wall thickness for this, can avoid either losing space on the interior, or having to add additional siding or trim boards of the exterior.
Don’t neglect to reflect on your needs; if it truly is more space that you need, or if a more functional arrangement of the space you already have could achieve the desired result. Sometimes usable space is better than more space.