Increased construction with timber is a potential way to improve environmental impact of the construction sector. If it is done right, timber is a renewable, sustainable and requires low amount of energy in the construction phase and during its service life. Vertical extensions of buildings are a potential way to increase a buildings relevant service life and to adapt the building to the increased needs of a city. It is also a way to contribute to densify cities with less resources compared to demolish the building and to construct a new one. Vertical extensions made with timber is light, compared to for instance concrete. This decrease the need for, or amount of, reinforcements needed in the existing building that is to be extended. There is a large unused potential in vertical extensions. Only a small amount of buildings is extended during its service life. The main purpose with this guide is to support developers and people that are involved in early stages of discussions and planning vertical extension of a building. It should also be useful as a support for technical people involved vertical extensions. Interviews with consultants and construction companies have shown that projects with vertical extensions is best run slightly different compared to normal projects were one start from “an empty plot”. In a project where you start from an empty plot, the developer and architects may normally be able to work rather free and independent from designers and heating, ventilation, and air conditioning engineers, in the early part of the project. For vertical extension projects the situation is different. The existing building possess special qualities and boundary conditions. For a successful project it is important to understand these technical possibilities and limitations. This is conveniently done by educate the developer in some of these parts, in order to understand what potential different buildings may have with vertical extensions and how some selections of concepts may complicate the extension. For the architect it is also important to work closely with the structural designer and the heating, ventilation, and air conditioning engineer from the beginning of the project, as a team. Experience have shown that not doing so may give concepts that is difficult to accomplish and needs for costly revisions. The first part of the process to decide if a building is suitable for vertical extensions is to investigate the condition of the existing building. Especially the load carrying capacity of the ground and the building, possible need for reinforcement and how reinforcements may be done. This is important understandings in the earliest parts of a project that usually need to be investigated or clarified. Building vertical extensions with timber may be done with a variety of building systems. Each have its benefits and limitations. Cross Laminated Timber (CLT) and post and beam building systems may be tailored in rather detailed dimensions adapted to the existing building. However, they require usually more installation work on building site compared to pre-fabricated modules. Post and beam are usually better for larger buildings. Pre-fabricated modules allows more of efficient pre-production in a factory but may require more framing structure and a higher connection part between the existing building and the extension. This due to modular systems tend to have more standard sizes which is not adapted to the existing building. In total, it is usually possible to achieve effective and aesthetical vertical extensions with timber building systems.