Home improvement additions or big upgrades should be considered when comparing similar homes you are considering making an offer on.
Cosmetic changes should be largely ignored, but major improvements should be taken into account. Most important would be room additions, especially bedrooms and bathrooms. Other items, like expensive floor tile or swimming pools should be taken into account too. A pool that costs $45,000 to install may or may not add that much value to the home.
Rely on your real estate agent to give you guidance on which home features add the most value to a home locally.