When you know you need to be flexible and that your project is due to go through lots of changes, limit the level of detail and/or complexity of your model: use "easy-to-update" geometry like Zones and Walls and avoid highly-detailed geometry like façade structures unless you have to build a geometrical model for visualization purposes. Simply use numerical model components to compensate for any missing geometry. The combination of key figures and simple geometry already provides you with excellent input for an estimate. With specific early phase calculation formulas (formulas are called "Recipes" and "Assemblies" in Vico's cost planning solution), you can generate detailed estimates with limited input. We even have a how-to webinar on the topic: Linking the Conceptual Estimate to the Model Progression Specification.
During the bid phase, when detailed design information is available and when most decisions have been made, you can add more complex geometry to your model. However, make sure that you know for which purpose you are doing this: define the quantities and/or properties that you want to extract upfront, so you do not spend time defining that curtain wall façade in a high level of accuracy: sometimes a representation by means of a simple wall already gives you with the quantity you need! (For example: surface area of the façade.)
Along with the design and specification documents of your project, the level of detail of your estimates will evolve over time. With model-based cost planning technology, you can simply replace the numerical or abstract geometry-based quantity input from the early phases with more detailed quantity data when it becomes available. With tools like Vico Office Cost Planner and Cost Explorer, you can even compare previous versions to current versions and perform target costing.
Once you have experienced the power that model-based estimating gives you (again: this can be either a geometric or a numerical model), you will find that you can be much more flexible and responsive in your work as an estimator. In fact, the estimator will be the first to see when and where a project is moving out of scope. And with visual communication tools (like the 3D model, the 5D cost plan, and a color-coded assembly structure) s/he can work with the project team to test alternative solutions by budget and schedule impact.
Having to spend significantly less time on quantity takeoff (when you go through iterations of alternatives, you can simply adjust your model's parameters) you can spend more time on what is most important in your job: managing cost and selecting the best solutions for the project, really using your knowledge of construction cost. Answering the "why?" questions during meetings will be easier: you know which model parameter was used as input for your cost calculations when changes to the design were made. With geometrical cost calculation input, you can even visualize where cost is coming from.
If you are convinced that model-based estimating makes sense, but you don't know where to start, just pick a project and start by enlisting the types of quantity data you typically use as input for your estimates. Then, take the estimating formulas and knowledge data that you already have and copy them to a model-based estimating application. Based on the required quantity data, build a simple 3D model - you will see that a lot of the quantities you need can be extracted from simple model elements like "Zones" and "Walls". For quantities that are missing, you can just define them as a numerical value - just replace them with geometry based data when you're ready for it. In no-time, you will have created your first model-based estimate. Again, we have a how-to webinar on the subject: Vico Office for Preconstruction and Estimating Teams.
By doing your first model-based estimate this way, you will get a feel for how it works; even when the only thing you're extracting from a geometrical model is "floor surface area." Also, you will start building your own company knowledge database, which you can re-use in your next project. You will experience that with every project, the amount of information that you can generate from models of limited complexity is increasing rapidly.
Over time, you will create templates for typical geometrical (for example typical wall and floor slab structures) and numerical (for example typical project key figures) models, which will provide you and your company with standard model based estimating guidelines. Predefined formulas ("Recipes" and "Assemblies") will make it easier for new colleagues to create accurate cost estimates, based on your company's knowledge database.
This 5D cost planning methodology works just as effectively in a hard bid scenario; and we have a how-to webinar on the subject. We've developed some Frequently Asked Questions for our Cost Planning solution. Please browse through them and let us know what other information you need to get started with model-based estimating! (And if you have an extra two minutes, head over to the Virtual Construction Comics for a humorous analogy about how Assemblies are like a recipe for baking a cake.)