The two separate CO₂ budgets in BR18

BR18 divides the climate calculation for construction into two completely separate accounts. This is not accidental — it is a deliberate design choice to prevent low material emissions from compensating for an inefficient construction process, and vice versa.

The two budgets are:

1. Budget 1 — Materials and operation: Covers modules A1-A3 (material production), B4 (replacement), B6 (operational energy), C3-C4 (waste treatment and disposal). The limit varies by building type.
2. Budget 2 — Construction process: Covers modules A4 (transport) and A5 (construction site activities). The limit is a flat rate of 1.5 kg CO₂e/m²/year, the same for all building types.

The crucial principle: you cannot borrow between the two budgets. Surplus on materials cannot compensate for exceeding the construction process limit. The two accounts are completely isolated.

Budget 1 covers the total climate footprint of materials from cradle to grave. The limit is differentiated by building type because different buildings have fundamentally different material needs.

Building type	2025 limit (kg CO₂e/m²/year)
Holiday homes and vacation properties under 150 m²	4.0
Single-family homes, row houses, holiday homes ≥150 m²	6.7
Apartments and commercial buildings (offices, retail, warehouses)	7.5
Institutions (schools, daycare, etc.) and other buildings	8.0

Limits tighten further in 2027 and 2029. Modules included in Budget 1: A1-A3, B4, B6, C3-C4. Module D is reported separately.

Module D (potential benefits): In addition to the modules above, module D must be calculated and reported. Module D covers potential climate benefits from reuse, recycling, and energy recovery after the building's lifetime. Important: Module D must be included in the calculation (cf. §297), but does not count towards compliance with the limit value (cf. §298). It is reported separately as information — it cannot be used to "save" a budget that would otherwise be exceeded.

Budget 2 covers transport of materials to the construction site (A4) and all activities on the construction site itself (A5). Here the limit is the same for all building types:

Year	Limit (kg CO₂e/m²/year)	Class
2025	1.5	Standard
2025	1.1	Low-emission class
2027	1.3	Standard (expected tightening)
2029	1.1	Standard (expected tightening)

The 1.5 kg limit was set based on analysis of construction site data from 2021. It is designed to force approximately half of construction sites to improve compared to the then-average. It is not an unrealistic ambition — but it requires deliberate effort.

The tightening to 1.3 (2027) and 1.1 (2029) means that what is today the "low-emission class" becomes the standard requirement within three years. Read more about the transition rules and effective dates.

For you as a builder, advisor, or documenter, the separation means concretely:

• Two separate calculations: You must perform two independent CO₂ calculations — one for materials (A1-A3 + B4 + B6 + C3-C4) and one for the construction process (A4+A5). They are not summed.
• Two separate data streams: Data for Budget 1 (EPDs, material quantities) and data for Budget 2 (invoices, fuel, waste) often come from different sources. In larger projects, however, it is typically the contractor who selects materials and can therefore also supply EPD data for Budget 1.
• No cross-compensation: A low material footprint (e.g. using timber) cannot save a construction process with high transport emissions. And conversely: an efficient construction site cannot compensate for emission-heavy materials. See the hidden CO₂ traps that can drive up the budget.
• Different actors: Budget 1 is typically managed by architects and material advisors. Budget 2 is managed by contractors and logistics managers. The two parties don't necessarily need to coordinate — but both must deliver.

The most common misunderstanding is that you have one combined CO₂ budget. You don't. And this mistake is most often discovered too late — at project completion, when it's too late to change anything.