A building permit is required for building a new building or increasing the floor area of a building. If renovation and alteration work affects the residents' safety or health, such work also requires a building permit.
Making significant changes to a building or its purpose requires a permit as well. The permit process involves assessing how the changes affect the city planning and land use, as well as how the requirements for the building change.
Small building projects require an action permit. Such projects can be various non-building structures and facilities or changes to the facade of a building. Geothermal wells and heat collectors require an action permit as well. The building ordinance of Lahti has relieved certain actions of the permit requirement.