At concept stage you estimate embodied carbon from quantities and generic factors, because the products that would carry EPDs haven’t been chosen yet. Take approximate material quantities from the massing, multiply each by an emission factor from a generic dataset, use a published method for building services, and report the result as a range with the life-cycle modules stated. That gives you a number good enough to compare options. It isn’t good enough to disclose, and it shouldn’t pretend to be.
This article sets out the method step by step, names the sources to use where no EPD exists, and lists the free and paid tools that do it for you. For the wider idea of comparing materials before a formal assessment, see what pre-assessment material intelligence is.
The short version
- Formula: Σ (quantity × emission factor). At concept stage, the quantities come from the massing and the factors from generic datasets.
- Generic datasets are legitimate. The EPBD calculation expects product EPDs where a product has been chosen and allows generic data where it hasn’t (see EPD vs generic dataset).
- Building services have no EPD for many products. CIBSE TM65 (January 2021) gives a “basic” method (A1, from product mass and material composition) and a “mid-level” method covering more stages.
- Report a range, say which modules are covered (A1–A3 only, or A–C), and show stored biogenic carbon separately.
- Use it to compare options (structure, retain vs replace, facade system), not to disclose.
- All figures here are indicative, for early comparison, not for certification or compliance.
What does a concept-stage estimate need?
Three inputs and one habit.
| Input | Where it comes from at concept stage | What to watch |
|---|---|---|
| Quantities | Massing model or sketch: slab areas, wall and roof areas, facade area, structural grid, storey count | Thicknesses and reinforcement ratios are assumptions. Write them down |
| Emission factors | Generic dataset per material (e.g. ÖKOBAUDAT generic datasets in Germany, baubook in Austria) | Declared unit and modules differ between datasets. Don’t mix them in one sum |
| Services | CIBSE TM65 where no EPD exists | Services are often left out of concept estimates. Say so if they are |
| The habit | Report a range and the module boundary | A single figure without a boundary reads as more precise than it is |
The calculation itself is simple. It’s the same everywhere: the mass of each material, multiplied by its factor, summed. What makes concept-stage estimates hard isn’t the arithmetic. It’s being honest about the assumptions.
Citation capsule: A concept-stage embodied-carbon estimate multiplies approximate material quantities from the massing by generic emission factors, uses CIBSE TM65 for building services where no EPD exists, and reports a range with the life-cycle modules stated. It’s fit for comparing options, not for disclosure.
Step by step
1. Fix the scope first. Which systems are in? Structure, envelope, internal walls, finishes, services. Which modules? Many concept-stage tools, Elementa included, cover the production stage A1–A3 only. The EPBD disclosure for new buildings covers the full life cycle to EN 15978 (see how modules A1–A3 and A1–A5 fit into it). Write the scope at the top of the result.
2. Take quantities from the massing. Floor slabs = gross floor area × slab thickness. Walls and facade from the perimeter × storey height. Roof from the footprint. Add a structural assumption (concrete frame, timber, steel) and a reinforcement ratio if concrete. Every one of these is an assumption; that’s fine, as long as it’s listed.
3. Pick one generic dataset and stay in it. In Germany, ÖKOBAUDAT’s generic datasets are what certification and funding routes expect. In Austria, baubook. Mixing datasets in one sum is the commonest way to produce a number that won’t reconcile later (see choosing embodied carbon software).
4. Handle services with TM65. Many mechanical, electrical and plumbing products have no EPD. CIBSE TM65, published January 2021, gives a “basic” method based on the product’s mass and material composition, focused on material extraction (A1), which it identifies as the dominant stage. It also gives a “mid-level” method that adds installation, maintenance, replacement and disposal stages (CIBSE TM65). If services are left out entirely, say so. They’re a large share on services-heavy buildings.
5. Keep stored biogenic carbon apart. Timber and bio-based materials carry negative production-stage values because of stored carbon. Compare on fossil emissions and report the storage on a separate line (see biogenic carbon in EPDs).
6. Report a range. The quantities are approximate and the factors are generic, so the result is a band, not a point. Show the two or three assumptions that move it most. Usually that’s the structural system, slab thickness and the facade.
7. Use it to compare. The value of a concept-stage number is relative: concrete against timber frame, retain against replace, one facade against another. The retain-or-demolish worked example shows how.
Citation capsule: A concept-stage embodied-carbon estimate follows seven steps: fix systems and modules; take quantities from the massing with stated thickness and reinforcement assumptions; use one generic dataset (ÖKOBAUDAT in Germany, baubook in Austria); estimate services with CIBSE TM65; report stored biogenic carbon separately; express the result as a range; and use it comparatively.
Is a generic-data estimate allowed?
For early design, yes, and it’s also how formal assessments begin. The EPBD calculation expects product-specific EPDs where a product has been chosen, and allows generic datasets where it hasn’t (see EPD vs generic dataset). At concept stage nothing has been chosen, so generic data is the correct choice, not a shortcut.
What isn’t allowed is presenting the estimate as the disclosed value. From 1 January 2028, new buildings over 1,000 m² must have their life-cycle GWP calculated in accordance with the Directive’s Annex III and disclosed on the energy performance certificate. From 1 January 2030 that applies to all new buildings (Directive (EU) 2024/1275, Art. 7(2)). That calculation is a full assessment. The concept estimate is what lets you know early what it will say (see pre-assessment vs certified LCA).
Which tools do this for you?
| Tool | What you enter | Cost | Notes |
|---|---|---|---|
| One Click LCA – Carbon Designer 3D | Building type, gross floor area, number of floors; the tool generates representative quantities | Business licence and above | Also supports “build new vs refurbish” comparisons (One Click LCA) |
| eLCA (BBSR) | Building components from templates | Free | Uses ÖKOBAUDAT; built for BNB; no in-depth LCA knowledge required (BBSR, nachhaltigesbauen.de) |
| baubook eco2soft | Building components | Free for up to 3 buildings (results watermarked “Draft”); €120 per user per year for professional use | Austrian; calculates GWP, AP, PENRE and the OI3 indicator (Energieinstitut Vorarlberg) |
| CAALA | From simple area inputs up to BIM | Paid | Connects to ÖKOBAUDAT; QNG and DGNB (gebaeudeforum.de) |
| Elementa | Existing and proposed building by system; a pathway per element | Open sign-up | Indicative, A1–A3, comparative, built for retain-or-replace decisions; not a certified LCA |
For a fuller comparison by job (certification, funding evidence, early design, sanity checks), see choosing embodied carbon software and One Click LCA alternatives.
Frequently asked questions
Can I calculate embodied carbon without EPDs?
Yes. Before products are specified, use generic datasets for materials and CIBSE TM65 for building services without EPDs. The EPBD calculation itself allows generic data where no product has been chosen. Replace generic values with product EPDs as the specification firms up.
How accurate is a concept-stage embodied-carbon estimate?
Accurate enough to rank options, not to disclose. The main uncertainty comes from quantities (thicknesses, reinforcement, structural system) rather than from the factors. So report a range, and show which assumptions move it most.
Which modules should a concept-stage estimate cover?
At minimum the production stage A1–A3, stated explicitly. The EPBD disclosure covers the full life cycle to EN 15978, so if you report A1–A3 only, say that it’s a partial scope. Keep stored biogenic carbon on a separate line.
Is Elementa’s estimate certified?
No. Elementa gives indicative, production-stage (A1–A3) figures for comparing options at concept stage, with the factors and assumptions labelled. A certified figure needs a full assessment by a qualified assessor.
The takeaway
A concept-stage estimate is quantities × generic factors, done honestly: one dataset, services via TM65, biogenic carbon apart, a range rather than a point, and the modules named. That number won’t go on an energy performance certificate. It will tell you, while the design can still change, which option to take.
If you want that estimate built per system, with retain-or-replace pathways for existing buildings and the assumptions visible, start a project.