I wanted to provide an overview of how to more accurately estimate remodeling/repair costs on a property that has no interior access, by using 3D modeling. This would be property that is currently occupied, out of state, or not allowed access for other reasons (common on auction.com for example). Instead of blindly placing an offer on a property, assess your risk more accurately by creating a simple 3D model of the house that you can use to derive repair costs.

What you'll need:

• SketchUp Make or SketchUp Pro - You can download SketchUp Make 2017 for free at sketchup.com/download/all. (You'll need to create a free Trimble account.) (Also note that SketchUp Make is no longer supported, but it will work for our needs here.) SketchUp Pro is also available with 30-day trial, then $299/YR or $695 one-time fee.
• (Optional) Quantifier Plugin $79- This SketchUp plugin makes it easier to assign cost in your model, but it's not necessary. You can manually extract area calculations from SketchUp to import into Excel or whatever to manually calculate your costs.
• Interior & exterior photos from the listing - We'll reference the interior photos inside SketchUp using the "Match Photo" tool, to check the 3D model and figure out where each room is located in the house. The match photo feature is the "magic" behind this whole workflow. Exterior photos are useful for getting the shape of the house correct, as well as matching windows from specific rooms with the interior photo to identify where a room is located in the house.
• Google Maps - 3D mode and street view offer additional perspectives of the house from the sky, as well as the street. (Hopefully no trees block the house, lol)
• County GIS - Most towns and counties offer online viewing of their GIS data, which typically includes a property card that has a basic sketch of the footprint of the home with rough dimensions. These dimensions will serve as a starting point to get the rough shape of the house.

The objective is to build a very simple 3D model of the house which can then be used to derive areas and volumes that can be used for estimating repair/remodeling costs more confidently, when you are denied interior access to the home and can't measure/inspect the interior space. Of course there is risk and assumptions with doing this. This post is not about that. I just wanted to offer a way to better assess a property that you can't access for whatever reason.  This isn't going to be a complete step-by-step, but instead, an overview of the process I came up with.

Step 1 - GIS

You'll want to start by finding out whatever dimensions you can get your hands on. Most towns have online GIS (geographic information system) available. You can try to find it by searching "<town>, <state> GIS", so, for example, "Warren, RI GIS".

Each GIS system will be different, but basically you'll see a map of the town, and you should have a search option where you can search by address, or you can simply zoom into the property manually. Once you find the property and click on it, there should be a "property card", which you can click on to view more info about the property. 

On the property card, you'll see assessment info, area calculations, and hopefully a rudimentary sketch of the footprint of the home, with rough dimensions.

3D Model in SketchUp

Using the footprint dimensions, you'll model a simple box in SketchUp. Use the Rectangle tool to create a rectangle, then type in the dimensions to set the rectangle to the correct size. Then, use the Push/Pull tool to extrude up 8' for now.

Next, you'll want to grab an exterior photo of the house to use the "Match Photo" feature with. The match photo feature enables you to change the camera perspective of SketchUp to match the camera perspective of the photo. This enables you to 3D model, while viewing the photo overlay, so you can basically build your model to match the photos. (Match Photo is a little tricky so I'm not going to cover it here.)

By creating multiple "Match Photos", you can compare several perspectives to your model, until you come up with something that matches the house fairly accurately.

You can also compare images from Google Map 3D view, as well as Street View, to give you a good idea of proportions. Sometimes the camera angle distortion messes with our perception, so it's always a good idea to view from multiple angles so you can adjust your model. In Google Maps, switch to Satellite view, then click the 3D button, then click the rotate view arrows to rotate around the property. 

At this point, you already have enough information to estimate siding and roofing replacement! Just select the surfaces you want to measure (hold CTRL while selecting with the Select tool to select multiple), and in the Entity Info window in SketchUp, it will show you the total area of those surfaces.

Interior layout

The interior is more tricky, because you typically won't have any dimensions, so you'll only be working off of images. In most cases, there will be areas of the house that aren't photographed at all, which can still be valuable because you can infer things that may uncover more risk. There's probably a reason they didn't take a picture, and this process will help you ask questions to the seller about specific areas of the house that other buyers won't even know to ask about.

You'll want to choose the first room by finding a unique window that you can identify in both the interior and exterior photo. This reference will help you locate exactly where the room is in the house. Most likely it will be a bay window, double window, or maybe a small bathroom window that is easily identifiable. You can also reference a chimney and fireplace easily, or any other unique object like a front door, or whatever. 

Another important thing to look for in the photos are perspectives that show open doors into the next room. By referencing the paint color of the adjacent room, you can pretty easily identify where the rest of the rooms are located. (assuming the whole house isn't painted white, lol)

Once you've got a room, you'll literally just look at the photo, and guess the sizes and placement of things such as walls, windows, and doors. Use commonly known dimensions to reference scale. For instance, baseboard is typically 3 1/2" tall. Doors are usually 6'-8" Tall. A stove is usually 30" wide. Things like that. Use those dimensions to help you infer other dimensions.

Using these tips, you can get your model looking something like this, with all of the interior spaces roughly defined.

Estimating costs

Now I use a SketchUp plugin called Quantifier to assign costs to things in my model. I like it because I can apply materials to the surfaces in my model, and it will calculate a cost based on the area of the surfaces that have the material applied. In the picture above, you'll notice that the exterior walls are green, and the interior walls are yellow. That's just because I use a different cost calculation for interior walls vs exterior walls, so the colors are different. Quantifier also allows you to assign costs by volume, layer, and by object. So there's a lot of flexibility.

If you don't want to use the plugin, you can just go to the Materials panel, click the Home button to view all materials in your model, then Right-click on the material > Area, and it will tell you the area of the surfaces that use that material. From there, you can plug that into a spreadsheet or share that with your contractor to get pricing.

Finding issues

Now remember when I said that some areas of the house won't have any photos? Well, in this house in particular, they don't show the stairs to the second floor. I figured out exactly where the stairs exist (by process of elimination), but there is no headroom to get up there! I can assume that whatever access there is to the 2nd floor, it's not up to code, and will likely be an issue that needs to be addressed.

You can go as detailed as you like with this, adding cabinets, appliances, "junk to be removed", etc. As many objects as you want to assign cost to. It's a much more accurate way to estimate cost and risk when you don't have access, even if you want to calculate a worst-case scenario. The visual aspect of having a 3D model is very helpful too for brainstorming ideas and solutions. 

btw, here's the property I used in this example https://www.auction.com/details/254-metacom-ave-wa.... Funny enough, interior access is granted (to realtors only), but I was just using this house as practice for this workflow.

Let me know if you have any questions!