Which Revit LOD Suits Extensions? Practical Guide

A proposed rear extension can look straightforward on a planning drawing, then become difficult the moment it meets an uneven boundary wall, a stepped roof, or an existing floor that is not level. That is why the question, “which Revit LOD suits extensions”, is less about selecting a number and more about defining the decisions the model must support.

For most extension projects, an accurate existing-condition Revit model at LOD 300 is the practical starting point. It provides dependable geometry for design development, planning information, coordination and drawing production without paying for modelling detail that does not yet serve the project. However, LOD 200, LOD 350 or LOD 400 can each be the right choice where the brief, procurement route or building complexity demands it.

What LOD means for an extension model

Level of Development, or LOD, describes how developed and reliable model elements are for a stated purpose. In a Revit model, this applies to elements such as walls, floors, roofs, windows, doors and structural components. It is not simply a measure of visual detail.

A highly detailed door family does not make a model suitable for coordination if its position, opening direction or surrounding wall geometry is uncertain. Equally, a simple wall representation can be highly valuable at an early stage if it accurately records the existing building footprint, levels and principal construction.

For extensions, the existing building model is usually the critical reference. The design team needs to establish where the new work meets the old, how floors and rooflines connect, whether head heights work, and where services or structure may constrain the proposal. The appropriate LOD should therefore be set against these design questions, not against an assumed standard of completeness.

Which Revit LOD suits extensions at each stage?

LOD 200 for feasibility and early option testing

LOD 200 is appropriate when an architect needs a reliable spatial framework rather than fully resolved construction geometry. Existing walls, floors, roofs and openings are represented in an approximate, recognisable form, with general size, location and orientation established.

This level can suit early feasibility work on a conventional extension, particularly where the team is testing massing, footprint, daylight implications or broad internal layouts. It is also useful where several options need to be assessed quickly before a preferred scheme is selected.

The trade-off is that LOD 200 may not give sufficient definition at junctions, around irregular openings or through complex roof forms. If the building is listed, heavily altered, or geometrically inconsistent, those missing details can become design risks rather than harmless simplifications.

LOD 300 for developed design and planning support

LOD 300 is generally the best fit for the majority of residential and small commercial extensions. Elements are modelled with defined size, shape, location and orientation, creating a dependable existing-condition base for plans, elevations, sections and coordinated design work.

At this level, a design team can work with meaningful floor levels, wall thicknesses, opening positions, roof geometry and principal structural features. It supports the practical questions that arise once an extension moves beyond an initial sketch: Can the proposed roof meet the existing eaves cleanly? Does the new threshold align with the internal floor? Will a side return extension affect window openings, drainage routes or access around the building?

LOD 300 does not mean every hidden component has been assumed or every material build-up has been specified. It means the visible and surveyed geometry is represented clearly enough for the agreed design purpose. This distinction keeps the model useful, proportionate and easier for the project team to manage.

For a standard extension, an LOD 300 model built from a high-accuracy measured survey or laser scan gives architects a strong platform for planning drawings, developed design and consultant coordination. It also reduces the need for internal teams to redraw uncertain existing information before design can begin.

LOD 350 where interfaces need closer coordination

LOD 350 is worth considering when the extension has complex interfaces between disciplines or difficult connections to the retained building. The model goes beyond individual elements to show how they interact, including relevant supports, connections and coordination zones.

This may be appropriate for a substantial commercial alteration, a basement extension, a heritage project with retained fabric, or a scheme involving steelwork, complex roof alterations and tightly coordinated services. In these cases, the value is not in producing a more elaborate model for its own sake. It is in making spatial conflicts visible before work reaches site.

LOD 350 requires a carefully defined scope. If the architectural survey model is expected to include every structural and MEP interface without consultant input, the result can be misleading. Each discipline should remain clear about who owns, verifies and develops its information.

LOD 400 for fabrication and installation information

LOD 400 is rarely the starting requirement for an extension survey model. It is intended for fabrication, assembly and installation-level information, often developed by specialist contractors or manufacturers once design responsibility and product selections are established.

There are exceptions. Bespoke joinery, heritage repair packages, intricate glazing, exposed steel or specialist roof systems may need fabrication-ready modelling at a later stage. But commissioning LOD 400 before those details are designed and agreed can introduce unnecessary cost and false precision.

The sensible approach is usually to begin with an accurate LOD 300 existing model, then develop only the selected elements to LOD 400 when the construction package requires it.

Accuracy and LOD are related, but not interchangeable

A common mistake is to treat LOD as a guarantee of survey accuracy. It is not. LOD describes the development of information in the model, while accuracy depends on the capture method, survey control, modelling tolerance and quality assurance process.

An LOD 300 model based on incomplete existing drawings can be neatly structured but still unreliable. Conversely, an LOD 300 model derived from registered laser-scan data and checked against site conditions can provide a very dependable basis for extension design.

This is particularly relevant in older buildings. Out-of-plumb walls, sagging rooflines, non-standard openings and historic alterations are not modelling errors if they reflect the building as found. They are conditions the extension design must accommodate. A precision-first survey and modelling process helps the team distinguish genuine irregularity from missing information.

Set the model scope before capture begins

The best time to choose an LOD is before the site survey, not after the Revit file is delivered. A short, clear brief can prevent both over-modelling and the more costly problem of discovering that essential geometry was not captured or represented.

Define the intended outputs and decisions first. If the model will support planning and architectural design, the scope may focus on the external envelope, principal internal walls, floors, roofs, openings, stairs and key levels. If structural coordination is expected, exposed structure, bearing relationships and relevant roof components may need clearer treatment. For services-intensive work, plant areas, risers, ceiling voids and visible service routes may also matter.

It is equally useful to record what is excluded. Hidden foundations, cavity construction, concealed steelwork and inaccessible roof areas cannot be confirmed solely through a measured survey. They should be identified as assumptions, exclusions or areas requiring further investigation, rather than modelled as certainty.

For listed buildings and complex existing properties, allow for a review of the point cloud or survey information before modelling is finalised. This gives the design team an opportunity to identify high-risk junctions, unusual features and areas where more detailed representation will genuinely improve the project outcome.

A proportionate model protects programme and budget

The right LOD for an extension is the lowest level that reliably supports the next decisions, with clear scope for further development where needed. For most projects, that is LOD 300 backed by accurate existing-condition capture. It creates structured, design-ready information without delaying progress through unnecessary detail.

Where interfaces are complex, LOD 350 can reduce coordination risk. Where specialist fabrication is confirmed, selected elements can move to LOD 400. Starting with dependable geometry allows those later decisions to be made from a sound base rather than from assumptions.

A well-briefed Revit model should make an extension easier to design, not harder to interpret. Establish the required decisions, capture the building accurately, and specify only the information that the project can use with confidence. Space Captures can help define that scope before survey work begins, so the delivered model supports the work in front of your team.