What to do with an existing building?

The situation in the past

Unless you are a company that only designs new buildings you come across and existing building once in a while. In the past you would ask the owner of the building for drawings or the city archives. When you were lucky there were digital CAD files for plans, elevations and sections, but when you were unlucky there were only drawings on paper or even worse, there was nothing at all and you had to measure the building and use pictures to find out how the building was put together.

If you had CAD files, you would clean them up a bit, get rid of unneeded info and make them comply with office standards. Took a few hours but that was it.

If there were only drawings on paper there was a bit more work as you had to do the entire drawings from stretch according to the paper drawings. You had to figure out which drawings you needed and recreate those. I don’t know if this is the same in other countries, but here in Holland the drawings from 20 years ago or so had much less annotations on them then is expected from us these days. One can ask himself why contractors in the past were able to build all those buildings with so little information and why the contractors these days want every nail to be detailed, but that’s another discussion. Usually it took days to figure out how thew building was put together, if you could even get all the info from the paper drawings. But after days of drawing you finally had a set of ‘as build’ drawings (or at least close to that) and you were off to start to draw the new design.

Current situation

These days in the Revit/BIM era, we still need to find drawings of a building we are redesigning and there is very little chance that there is a 3D model of the building in any sort of file format let alone one that is useful. Question is if a model made by a 3th party is something you would be happy with, but that too is a different discussion.

If there are just paper drawing or CAD drawings you have about the same issue, you need to convert those to a 3D model, or is there another way?

In my work I came across all the options and in the end it all comes to common sense and thinking before you start to model stuff. To me there are basically 3 questions you need to ask yourself before you start to do anything:

  • What do they pay me for?
  • What am I going to do with the existing building?
  • What do other people involved expect from me?

What do they pay me for?

This is the easiest of the 3 questions. Did you agree with the owner of the building that you will make a detailed 3D model of the building, than you should do that as you should have taken that into account when made the bid for the work. When you are doing a BIM project then to a 3D model of the existing building is needed.

Did you agree to just deliver 2D drawing in DWG, PDF or paper then it is a different story. In that case Revit is just a tool to get the job done as good, fast and efficient as possible and the 3D model is not one of the goal, but just a means to get to what you need.

I believe that there are 3 different types of projects with existing buildings and all can have different way to be handled when you just need to deliver 2D drawings:

  1. Interior design
  2. Internal remodeling
  3. External remodeling

 Interior re-design


With minimal construction work creating a new interior

For me there are 2 options. If there are CAD drawings you can clean those up and use them as an underlay in Revit and add 3D walls and furniture to it. To be able to use Rooms you will have to add Room Separation Lines. This will give you a nice result in a short amount of time. Are there no CAD drawing you will have to model the building yourself and of course that should be done in Revit to be able to use all advantages that Revit has. For an interior project this existing model can be relatively simple, use 2 Wall Families, one for existing walls and one for load bearing walls and use curtain walls for windows and doors. Also model the building on a floorplan level, meaning there is no need for structural beams that you don’t see anyway on your floorplans and the same goes for floors and roofs. Doing it like this you can use the advantages of Revit and have yourself an existing model in no time flat.

Interior reconstruction


Doing mostly interior work, but with a few modification on the building itself.

This is where the use of CAD underlays is not possible anymore, because you will be needing existing drawing, new drawings and possibly a drawing that shows what has to be demolished. Most likely you will be needing sections as well. This means you can model a building about the same as I mentioned above, but with floors and roofs to make the sections accurate. And in this case there are a few choices again. Are you removing floors you will have to model the structural beams that are around that floor, the question is, will you only model those at the location where you are actually doing structural work or will you model them throughout the building? I guess that depends on how accurate you wanna be and what the owner of the building wants. The same goes for smaller items, like sills and roof trimmings.

Complete building redesign



Stripping the building to its bear structural parts and redesigning a new building around them.

With a complete remodeling of a building you will of course need both new and existing drawing for elevation, sections and plans and the only thing you can do for these projects is to model the existing building. You will also need to take a good look at structural beams and columns for a project like this, so those will need to be modeled as well. As this is also a redesign of the exterior of the building, the existing model would need to be a bit more detailed to be able to show what is changing. This means that stuff like roof trimmings and concrete wall sweeps needs to be modeled to be able to show they are removed or to use them in the new design. Once the existing model is done the full array of tools that Revit has to offer can be used for the new model.

What do other people involved expect from me?

Well, this is a tricky one and a simple answer could be, “what do I care about other people, the owner of the building just wants PDF drawings and he is the one paying me, so why model the existing building in detail, contractors were able to make buildings in the past to without a 3D model”. In a way this is true, a contractor was able to make buildings with just paper drawing back in the days. But instead of seeing the modeling of a building as un-payed extra work you could try and marked the model. Is the contractor prepared to pay a little extra money for the model as it makes his life a lot easier, or is the owner of the building prepared to pay a bit more money so that the contractor and other parties involved have an easier job and save him some money in the end. Or are you prepared to invest a little extra time and money to make a good model so that you will not have surprises during construction and save time there as thing do get clear sooner in a 3D model compared to 2D drawings. Not having to solve all sorts of issues during construction saves everybody money in the long run.

So, it seems there is no definitive right way to do all this, but there are a few things you need to think about before starting a project and I believe that the choice will be different for each project.

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