||This paper describes an ongoing research project on application of a 4D CAD tool for design and production planning of vertical extensions of existing buildings (over-roofing) in Stockholm city, for creation of a more densely populated city as the demand for apartments in the city centre increases.4D CAD is a concept, which combines an object oriented 3D CAD model with time. 4D CAD is a kind of information visualisation that is easier to understand than traditional methods, such as 2D drawings and time schedules, which are used to manage construction projects. 4D CAD is a logical way of imaging a construction management tool. It is a tool that is conceptually much closer to an intuitive picture of a construction process than 2D drawings and time schedules.The 4D concept is developed at Stanford University and to support the concept researchers at Stanford have developed a prototype that is being used in some complex construction projects in California.The focus of the research project “Integration of apartments in existing buildings by use of Light Gauge Steel Framing”, which this paper is a part of it, is to improve production methods in order to reduce design, planning and construction time for conversions of, and extensions to existing buildings in the city centres. A way to improve the production methods is by utilising a 4D planning process in combination with industrialised production of building components.Extensions to existing buildings are due to the demand for new apartments in attractive locations in the city centres and shortage of land for housing in city centres.The Light Gauge Steel constructions have many benefits for conversions of, and extensions to existing buildings. According to research results the Light Gauge Steel Framing system is suitable for industrial production. This building system results in a very light weight building compared with traditional materials e.g. concrete which makes it suitable for over-roofing extensions. The materials used in the Light Gauge Steel Systems is thin steel members, plaster boards and mineral wool.Many of the problems, which occur during vertical extensions of existing buildings today, are solved when they are discovered, that is sometimes on the site. Some examples of these problems are poor compatibility between the existing building (structural components and material) and the Light Gauge Steel Framing, detail solutions of the building components, shafts and piping for ventilation, water, sewage and drainage etc. It is less expensive to discover and to correct errors at an early stage compared to solving them on the site. Further a lot of construction time will be saved, which will decrease the disturbance on existing surroundings.Several problems have to be considered in the planning process in order to minimise the disturbance on existing activities and surroundings. This could be done by the use of a 4D CAD planning tool.An over-roofing project located in the city means that the land to use during the production period is limited and expensive. Thereby is the logistic to and from the site more complicated. Consequently the site management and logistic of building components to the building site and their storage on the site is most important.In fact the 4D concept is an efficient planning tool to organise the logistic of the site during the planning phase instead of as today during the production. The site layout can be simulated and visualised with a 4D CAD tool for the actors in the project which in particular will help the site engineer to organise the activities, material flow and site logistic.The value of using the 4D CAD concept is studied by comparing the traditional planning process of a number of over-roofing projects in Sweden with the planning process of the 4D CAD concept. This paper discusses how a 4D CAD tool together with an industrialised production method can be used for improving the production process for an over-roofing project in order to reduce the construction time and with secured quality. The reader will understand and appreciate the added value in form of a more efficient way of managing construction projects.