Permits, air and water EWSNI
Medium combustion plant and specified generator permits
Any boiler, engine or turbine with a rated thermal input of 1 MW or more but less than 50 MW is a medium combustion plant (MCP) and needs an environmental permit controlling sulphur dioxide, nitrogen oxides and dust. New plant has needed a permit since 20 December 2018; existing plant of 5 to 50 MW since 1 January 2024; and existing plant of 1 to 5 MW must be permitted by 1 January 2029. Generators face extra "specified generator" rules.
At a glance
- Who must comply
- Operators of boilers, engines, turbines, generators, CHP units and furnaces of 1 MWth to under 50 MWth: hospitals, universities, factories, data centres, district heating, estates and standby power sites.
- Thresholds
- 1 MW rated thermal input per unit (net calorific value); new units sharing a stack are aggregated. Over 50 MW the large combustion plant and UK ETS rules apply.
- Next key date
- 1 January 2029: existing 1 to 5 MW plant must hold a permit (and comply with emission limits by 1 January 2030). Applications open in advance; apply by early 2028.
- Regulator
- Environment Agency (England), Natural Resources Wales, SEPA under EASR (Scotland), NIEA (Northern Ireland).
- Maximum penalty
- Operating without a permit: unlimited fine and up to 5 years' imprisonment on conviction in England and Wales; uncapped variable monetary penalties in England.
- Where to do it
- Environmental permit application to the Environment Agency, NRW, SEPA (EASR) or NIEA; standard rules permits are available for most MCP in England.
This probably applies to you if
- You run a gas or oil boiler of 1 MW or more (roughly a building of 10,000 square metres or more, or a process plant)
- You run standby or peaking generators of 1 MW or more, including diesel back-up sets at data centres and hospitals
- You run CHP engines, gas turbines or biomass boilers between 1 and 50 MW
- You have several new units sharing a chimney whose combined input reaches 1 MW
- You are buying a site with boiler plant installed before 20 December 2018
What counts as a medium combustion plant
The rules, which transpose the EU Medium Combustion Plant Directive into the Environmental Permitting Regulations, apply to each combustion unit with a rated thermal input of 1 MW or more and less than 50 MW, whatever the fuel and however many hours it runs. Thermal input is measured on the net calorific value of the fuel. Units under 1 MW are not counted, but new units on one site sharing a common stack or windshield are added together, and operators must not split discharge points to avoid aggregation.
| Classification | Definition |
|---|---|
| New MCP | Put into operation (fuel first burned) on or after 20 December 2018 |
| Existing MCP | Put into operation before 20 December 2018; becomes "new" if substantially refurbished (over 50% of the cost of a comparable new plant) or altered so that its emission limits would change |
| Specified generator | A generator producing electricity, classed as Tranche A or Tranche B depending on when it was commissioned and its contracts, with its own emission limits and deadlines |
Excluded plant includes large combustion plant and incinerators (regulated separately), mobile plant covered by non-road mobile machinery rules, direct-fired dryers where combustion gases contact the product, flares, crematoria, pulp recovery boilers, offshore engines and research or test-bed engines.
Permit deadlines
| Plant | Permit in place by | Comply with emission limits by |
|---|---|---|
| New MCP | Before commissioning (since 20 December 2018) | From commissioning |
| Existing MCP, more than 5 MW to under 50 MW | 1 January 2024 | 1 January 2025 |
| Existing MCP, 1 MW to 5 MW | 1 January 2029 | 1 January 2030 |
The Environment Agency has said it will open applications for the 1 to 5 MW tranche well before the deadline; it recommends applying at least a year ahead so the permit is issued in time. If your 1 to 5 MW plant generates electricity it may already need a permit as a specified generator under earlier deadlines, and the stricter rules apply.
Watch out: "put into operation" means the date fuel was first burned in that unit. Keep commissioning records traceable to the serial number; without them the regulator may treat the plant as new, which brings tighter limits.
Specified generators
Generators that produce electricity are regulated more tightly because of their impact on local air quality. The specified generator rules apply to generators up to 50 MW and set nitrogen oxide limits and deadlines by tranche: Tranche A generators (broadly those operating or with capacity agreements before December 2016) had to be permitted in stages to 2025 or 2030 depending on their air-quality impact; Tranche B generators (commissioned or contracted later) need a permit before operating. Back-up generators that run only for testing and emergencies for less than 50 hours a year are generally excluded from the specified generator rules, but still count as MCP if they are 1 MW or more. Where both regimes apply the stricter emission limit and the earlier deadline win.
Emission limits, monitoring and records
Permits set emission limit values for sulphur dioxide, nitrogen oxides and dust, which depend on fuel, plant type, size and whether the plant is new or existing. Plant running fewer than 500 hours a year (averaged over five years) can be exempt from some limits. Operators must:
- monitor emissions periodically (every three years for plant under 20 MW, annually for 20 MW and above, unless the permit says otherwise);
- keep records of operating hours, fuel used and any malfunctions for at least six years;
- record and report breaches and follow the permit's conditions on maintenance and stack height.
How to apply, step by step
Inventory your combustion plant
Every boiler, engine, turbine and generator with its rated thermal input, fuel, commissioning date and stack arrangement.
Classify each unit
New or existing MCP, specified generator tranche, or excluded. Note the permit and compliance deadline for each.
Choose the permit route
Standard rules permits cover most low-risk MCP and generators in England; bespoke permits are needed for plant in air quality management areas or near sensitive habitats, and for all MCP in Wales.
Gather the evidence
Site plan, stack heights, manufacturer emissions data, an air quality screening assessment where required, and your management system.
Apply and pay
Submit to the regulator with the application charge; budget for the annual subsistence charge once the permit is issued.
Set up monitoring and records
Periodic stack testing by an accredited contractor, operating hour logs and a maintenance schedule.
Scotland, Wales and Northern Ireland
Scotland regulates MCP under EASR; SEPA has set application windows for different plant types, including combustion plant generating electricity in 2028. Wales follows the same deadlines as England under the 2016 Regulations, but NRW no longer issues standard rules permits for MCP, so applications are bespoke. Northern Ireland transposed the directive into its own regulations with the same 2024 and 2029 deadlines, administered by NIEA. See our environmental permits guide for charges and the general application process.
How it differs across the UK
- E England
Regulated by the Environment Agency under Schedule 25A and 25B of the 2016 Regulations; standard rules permits available for most plant; existing 1 to 5 MW plant permitted by 1 January 2029.
- W Wales
Same deadlines under the 2016 Regulations, regulated by Natural Resources Wales, which issues bespoke rather than standard rules permits for MCP.
- S Scotland
Regulated by SEPA under EASR since 1 November 2025; SEPA sets application windows by plant type, including electricity-generating plant in 2028.
- NI Northern Ireland
Regulated by NIEA under the Pollution Prevention and Control (Industrial Emissions) (Medium Combustion Plant) Regulations, with the same 2024 and 2029 deadlines.
Key dates
-
EASR application window opens for combustion plant of 1 MW or more generating electricity on site in Scotland (closes 30 June 2028)
-
Existing medium combustion plant of 1 MW to 5 MW must hold an environmental permit
-
Existing 1 MW to 5 MW medium combustion plant must comply with emission limit values
Dates marked “Proposed” are announced or consulted on but not yet law. See the full deadlines calendar.
Penalties and enforcement
| Breach | Penalty | Applies in | |
|---|---|---|---|
| Operating a medium combustion plant or specified generator without a permit, or breaching permit conditions | Unlimited fine and up to 5 years' imprisonment on conviction; variable monetary penalty with no upper limit in England | EW | Source |
| Carrying on a regulated activity without an EASR authorisation | Fine up to £40,000 and/or 12 months on summary conviction; unlimited fine and/or 5 years on indictment | S | Source |
Compliance checklist
Where to do it: official services
-
Medium combustion plant: when you need a permit
Guidance
Definitions, deadlines, exclusions and how to classify plant. Environment Agency via GOV.UK E
-
Medium combustion plant and specified generators: environmental permits
Register or apply
How to apply, which permit type to use and how the rules interact with installations. Environment Agency via GOV.UK EW
-
Find out if you need a permit for your boiler, engine or turbine (Wales)
Guidance
NRW's guidance and application route for MCP and generators in Wales. Natural Resources Wales W
-
Medium combustion plant guidance (Scotland)
Guidance
SEPA's EASR guidance for medium combustion plant. SEPA S
The law behind this
Questions people ask
Does a 1.5 MW gas boiler need an environmental permit?
Yes. A boiler of 1 MW or more thermal input is a medium combustion plant. If it was commissioned on or after 20 December 2018 it should already have a permit; if it is older, you must have a permit by 1 January 2029 and meet the emission limits by 1 January 2030. The Environment Agency recommends applying during 2028 at the latest.
Do standby generators need a permit?
Back-up generators that run only for testing and emergencies for under 50 hours a year are generally outside the specified generator rules, but a generator of 1 MW or more is still a medium combustion plant and needs an MCP permit by the relevant deadline. Generators that run commercially (for example under capacity market or demand response contracts) are specified generators and need a permit before operating.
What is the difference between new and existing medium combustion plant?
New plant first burned fuel on or after 20 December 2018 and has needed a permit from day one, with stricter emission limits. Existing plant was commissioned before that date and is permitted in two tranches: 5 to 50 MW by 1 January 2024 and 1 to 5 MW by 1 January 2029. Substantial refurbishment can turn existing plant into new plant.
How often must medium combustion plant be monitored?
Typically every three years for plant under 20 MW and every year for plant of 20 MW or more, by an accredited contractor, with the first test within four months of the permit or commissioning. Your permit sets the exact frequency and parameters. Keep operating hour logs and fuel records for at least six years.
Can I add several small boilers together to stay under 1 MW?
Units under 1 MW are not counted individually, but new units on the same site that share a stack or windshield are aggregated, and the regulator can treat deliberate splitting of discharge points as avoidance. Existing plant is not aggregated. Get the regulator's view in writing before relying on separate stacks.
Terms used in this guide
- Rated thermal input
- The maximum fuel energy a combustion unit can burn, in megawatts, based on the fuel's net calorific value; the measure used for the 1 MW and 50 MW thresholds.
- Specified generator
- An electricity-generating combustion plant subject to additional nitrogen oxide limits and permit deadlines under Schedule 25B.
- Tranche A and Tranche B
- Categories of specified generator based on commissioning date and capacity agreements, which determine permit deadlines and emission limits.
- Emission limit value (ELV)
- The maximum concentration of sulphur dioxide, nitrogen oxides or dust a plant may emit, set in its permit.
Sources
- Medium combustion plant: when you need a permit, GOV.UK (updated 9 July 2025)
- Medium combustion plant and specified generators: environmental permits, GOV.UK
- Find out if you need a permit for your boiler, engine or turbine, Natural Resources Wales
- Medium combustion plant guidance, SEPA
- Moving to the new regulations (EASR), SEPA
- Directive (EU) 2015/2193 on medium combustion plants, Article 5
Reviewed 14 October 2026. If you spot something out of date, email info@thamesenvironment.com.