You have a compliance calculation, not a prediction
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Approved Document L tells you, in its own text, that the number you
are holding is not a prediction. Volume 2 of the edition in force today,
at paragraph 9.4, carries this note: “The compliance outputs of SBEM or
other Building Regulations compliance tools are not suitable for direct
use as energy forecasting estimates for any size of building.” The 2026
edition, which takes effect on 24 March 2027, repeats it at paragraph
8.3. If the only energy figure in a building’s file was produced to
satisfy Building Regulations, you do not have an estimate of how that
building performs. You have a pass mark, and the document that issued it
has disowned it as a forecast in advance.
The
calculation was answering a different question
This is not a drafting accident. A compliance calculation compares
the actual building against a theoretical notional building of the same
size and shape with standardised properties. UKGBC’s Design for
Performance guide describes that approach as measuring performance
“against a variable baseline (i.e., a relative assessment)”, as opposed
to an “accurate assessment of the absolute energy consumption”. The
calculation is a comparison, and it was built to be one.
The consequence is that a building can pass comfortably and still
consume far more than anyone expected, without anything having gone
wrong in the modelling. The modelling was never asked. The most quoted
UK statement of this is a February 2012 briefing from the CIBSE Energy
Performance Group, written by Anna Menezes, which put it plainly: these
calculations “are commonly misinterpreted as predictions of in-use
energy consumption, when in fact they are simply mechanisms for
compliance with Building Regulations.”
Two cautions about that briefing, because you may be shown it. Its
headline figure, that buildings “typically consume 2 to 5 times more
energy than predicted at design stage”, rests on PROBE reviews of 23
buildings featured as exemplar designs between 1995 and 2002, together
with Carbon Trust programme findings. That is 1990s and early 2000s
data, and anyone quoting the multiplier at you in 2026 without saying so
is quoting a number older than most of your BMS. The briefing also
carries a note that it “does not necessarily reflect the views of
CIBSE”. The mechanism it describes has aged far better than its
arithmetic, and the mechanism is what matters: compliance modelling
deals with regulated loads, while unregulated ones such as small power,
server rooms and external lighting “typically account for more than 30%
of the energy consumption in office buildings”.
Part L never
asks whether the number was right
Here is the part that decides the argument. Approved Document L does
ask for a forecast of actual energy use in kWh per year, including all
metered energy uses and unregulated loads, prepared by a method such as
CIBSE’s TM54. It asks for it only for new buildings with a total useful
floor area over 1000m2, only as information for the log book, and in the
language of guidance rather than duty.
For the building you already own, already occupy, and are not
currently altering, no such forecast is required, because Part L bites
on building work. Approved Document L asks for no measurement of one
either. The regime that produced your number has no mechanism for
checking that number against reality, and it does not claim to. That gap
is not a loophole. It is the design.
The rating
you are managing against is itself moving
If the answer is to manage against the certificate instead, note what
is happening to the certificate. The government’s partial response on
Energy Performance of Buildings reform, published on 9 March 2026, keeps
a single carbon-based Environmental Impact Rating as the headline
non-domestic metric and states that, “subject to parliamentary
approvals, we intend to bring forward regulations in 2026”. New EPCs are
being worked towards from October 2026. Display Energy Certificate
validity periods were not settled in that response at all and were
deferred to a further one. A rating mid-reform is a compliance
obligation to track. It is not a management metric, and treating it as
one is how estates end up defending a band rather than a building.
The
scheme funded the plant far more often than the meter
The Public Sector Decarbonisation Scheme’s final phase awarded 245
grants to 207 organisations, totalling £816.6m. Its technology table
counts the projects installing one or more of each measure: air source
heat pumps in 135 projects, building energy management systems in 37,
and meters in eight.
Read that precisely, because it is easy to overstate. It counts
meters bought with grant money, not meters in existence, and many
recipients will already have had them. What it does show is what the
money went on when a national scheme paid for the shopping list. The
plant was the project. The instrument that would tell you whether the
plant worked was, for most of them, somebody else’s line item.
What to do, and what it
will not fix
The measured answer exists and it is narrow. NABERS UK Design for
Performance commits a client to a target rating in use, verified by 12
months of representative energy consumption data. UKGBC is explicit that
“only office buildings are currently eligible for NABERS certification”,
though the principles travel. No equivalent certification binds a
hospital, a campus or a school. Display Energy Certificates are the
nearest instrument in this territory for public buildings, and what they
currently require is worth confirming against the regulations rather
than assuming, particularly as their validity periods are among the
questions the March response deferred. Absent a scheme, the workable
move is to adopt an absolute metric such as Energy Use Intensity, meter
to it, and hold a full year before drawing conclusions.
Now the part you did not want. That year is real, so a decision you
need to make in March will not be waiting for your data. Metering costs
money and saves nothing on its own. And the first year of measurement
will usually make your business case look worse rather than better,
because it replaces an estimate that flattered the building with a
figure that does not. The government’s own evaluation of PSDS Phases 3a
and 3b records, under the heading of energy price rises, that “following
project completion, several Phase 3a and Phase 3b grant recipients
described how running costs had increased and in at least one case this
was felt to have affected stakeholder appetite for further works”. The
evaluation does not separate tariff from plant. Neither can you, without
data on your own building.
None of this makes the investment wrong. It makes the confidence
unearned. The choice is not between a good number and a bad one. It is
between knowing what your building does and being told what it should
have done.
