2D vs. BIM: The Battle for Accuracy in Construction Estimates and How to Save on Costs
March 5, 2024 at 7:00 am,
No comments
Incorrectly estimated construction quantities and, consequently, costs, cause significant stress for both contractors and clients. As I drive around my city, I notice several unfinished houses—partially built fences, missing external wall finishes, and other incomplete elements. Often, the root cause is the initial underestimation of construction costs, leading to the accumulation of additional expenses. This dilemma forces a difficult decision on which tasks to forego and which ones to complete.
Building Information Modelling (BIM) is a process that, if executed correctly, can substantially enhance the accuracy of estimates during the design phase and diminish the likelihood of unexpected expenses during construction.
If you wonder what BIM is - read this post first: What is BIM and Why Should I Care?
Comparison Between 2D and BIM Estimation
In the table below you can see the key aspects of estimation and a comparison of the similarities and differences between a classical approach based on 2D drawings and estimates made from BIM.
2D Estimation | BIM Estimation | |
Estimation Process | Quantities are typically derived manually from 2D drawings, such as architectural, structural, and MEP (Mechanical, Electrical, Plumbing) drawings. The level of detail in these drawings varies, meaning that information can be missing, and quantities need to be estimated based on visual interpretation and measurements which result, quite often, in human error. | BIM models (if they are available) contain detailed information about building elements in a digital format. These models include 3D geometries, material properties, spatial relationships, and other attributes. Quantities can be automatically extracted from the BIM model with high accuracy due to the richness of information available. |
Information Availability for Estimation | Drawings are legally mandated for construction projects, although for smaller buildings, certain disciplines such as plumbing or electrical work may not necessitate drawings. Additionally, the content of the drawings can vary significantly. During the design phase, drawings are typically only accessible at the end of milestones. | Building Information Models (BIM) remain largely uncommon, particularly in small-scale construction. However, the hindrance is not primarily the cost but rather the lack of demand. Similar to drawings, even if some models are available, certain disciplines might be absent. The same principle applies to modeled objects—not everything can or should be modeled. |
Accuracy | The accuracy of quantities derived from 2D drawings is prone to errors and inaccuracies due to the manual nature of the process and the potential for misinterpretation of drawings. | BIM facilitates a more accurate quantification process, as quantities are derived directly from the digital models. The information within the BIM model is consistent and can be updated in real-time, reducing the likelihood of errors. |
Speed | The quantification process is performed manually by counting each object in each drawing. This method is considerably slower when compared to using models. | With sufficient experience, extracting quantities from models becomes a rapid process. Moreover, since it is not imperative to have drawings for this extraction, it is possible to generate estimates more frequently, such as during the early concept stages. |
Skills | Reading drawings and estimating from them requires a good understanding of various graphical representations of elements. | Extracting quantities from models necessitates the use of specific tools. The difficulty of these tools varies, and the accuracy of the quantities also depends on the quality of the models. Evaluating the quality is not easy. Nevertheless, extracting quantities from models does not require an in-depth knowledge of graphical symbols, and if the objects can be identified, then the process is easy to learn. |
Additional Benefits | Even if only some models are available, the visual representation of the building and the ability to create 3D sections anywhere allow for a better understanding by the estimator and result in higher quality estimates. Quantity take-off is no the only BIM use. There are plenty of others, that can help save costs in small scale construction. See more BIM benefits I utilized during construction of my home in this video I made: The Value of 3D Models During the Design of Your Home |
In summary, while both 2D drawings and BIM models can be used to derive construction quantities, BIM offers significant advantages in terms of accuracy, efficiency, interoperability, and visualization capabilities. In small housing models are still rarely developed, but in THIS ARTICLE I explain that it is up to you as the client to ask this and it can be the same cost as 2D.