Yes, you can extend a Green Belt house in Hertsmere if the addition is not disproportionate to the original building, on its own or with earlier extensions, and stays subordinate to it. That is Policy SADM26 of Hertsmere's Site Allocations and Development Management Policies Plan, adopted in 2016 (SADM Plan, Policy SADM26).
On this page
- What does SADM26 say about extensions?
- Which five things does the officer compare?
- What is the "original building" in Hertsmere?
- What about replacing the house?
- Is a house in a village envelope outside the Green Belt?
- Do I need planning permission at all?
- What should the application show?
- How long does Hertsmere take to decide?
- Frequently asked questions
Hertsmere sets no percentage. Instead, the policy names five things the council compares between the existing and proposed development. Knowing them is how you design something that passes.
What does SADM26 say about extensions?
SADM26 opens with seven design principles for all Green Belt development: keep it unobtrusive, group buildings together, retain open and garden space, keep scale, height and bulk sympathetic to the landscape, use local materials, keep trees and hedges, and protect farm holdings. Then it controls scale:
- a replacement building in the same use must not be materially larger than the one it replaces;
- an extension or alteration "must not individually or cumulatively be disproportionate to the original building"; the additions must be subordinate to the original.
Those two lines follow chapter 13 of the National Planning Policy Framework (NPPF, GB7(1)(b)). The word "subordinate" is Hertsmere's own and matters: an addition can be modest in size and still fail if it competes with the house.
Which five things does the officer compare?
In judging scale, SADM26 says the council compares existing and proposed development by:
- floor space;
- volume;
- site coverage of building and hardstanding;
- height and orientation; and
- the use and its intensity.
The third is the one owners miss. A new driveway, a large terrace or a parking court adds to site coverage, and the council counts hardstanding alongside buildings. Plan the landscaping with the extension, and consider removing surplus hard surfaces to offset it.
What is the "original building" in Hertsmere?
The policy footnote defines it as the building in existence on 1 July 1948 or, if built later, as built. Two Hertsmere details follow from the plan's glossary and footnote:
- Outbuildings over 5 metres away are not part of it. A detached 1930s garage at the end of the drive cannot be added to the original house to enlarge the base.
- A basement can count. The footnote says the definition of extension, original or replacement building "may include a basement level". Do not assume a basement is free.
Every permanent addition since 1948, or since the house was built, counts against the original.
An illustrative example, not a real job: a 1950s detached house in Ridge has a 1980s double garage joined to its side and a wide gravel forecourt. A proposed two-storey rear extension is set below the ridge. The officer will compare floor space and volume against the original house, so the garage already counts. Reducing the forecourt and keeping the extension single storey on one side improves both the site coverage figure and how subordinate the addition looks.
What about replacing the house?
SADM26 says a replacement building in the same use must not be materially larger than the one it replaces, and the same five comparisons apply. The policy adds a warning: "A like for like replacement in terms of building is not necessarily acceptable." The nature and intensity of the new use, its effect on amenity and landscape, and the purpose of the Green Belt in that locality are also weighed. So a replacement is not a way around the extension test, and a new house twice the size of a modest bungalow is unlikely to be accepted.
The policy also covers limited infilling and redevelopment of previously developed sites, which must have no greater impact on openness than the existing permanent development. That is the test for a site with old garages or stores on it, not for an extension.
Is a house in a village envelope outside the Green Belt?
No. Policy SADM23 draws village envelopes for parts of Elstree, Shenley and South Mimms, but says the areas inside them "are part of the Green Belt". The envelope allows limited infilling of new houses; it does not lift SADM26 for extensions. Our Shenley post and Elstree post cover those villages.
Do I need planning permission at all?
Not always. The Green Belt does not remove householder permitted development rights (GPDO 2015, Schedule 2, Part 1). A rear extension within the national limits, or a loft within the volume allowance, may need no application and no SADM26 comparison.
Question 1 of up to 4
What type of property is it?
- Property
- Not answered yet
- Area
- Not answered yet
- Work
- Not answered yet
- Route
- Not answered yet
Check the planning history: rights are often removed by condition on newer houses and earlier extensions, and in a conservation area side extensions and some roof work lose them. If your project qualifies, no planning application is needed, and a certificate of lawfulness proves it.
What should the application show?
The officer will make the five comparisons whether you help or not, so it is better to do them first. We put a schedule on the drawings giving original, existing and proposed floor space and volume, a site plan with building and hardstanding areas before and after, and elevations showing heights. Where the original house is unclear, we say how we identified it from the planning register and the building.
SADM26's design principles count as much as the figures. The policy asks for development to be "as unobtrusive as possible", to keep existing open and garden space, and to keep trees and hedges. A site plan showing retained trees, and an extension tucked behind the house rather than projecting towards a lane or field, answers those principles directly.
If the figures already show the house has grown a lot since 1948, we say so before design starts, and look at permitted development or internal changes instead.
How long does Hertsmere take to decide?
Hertsmere decided 224 of its 246 householder applications within 8 weeks without an agreed extension of time, from July 2025 to June 2026 (MHCLG PS2).
See our Hertsmere page and Hertfordshire page. For the drawings, see our house extensions service.
Work out your fee
FFP-01 · Rev AHouseholder application fee, paid to your council at submission.
Prices include one measured survey of a house of up to two storeys (£250). A flat or maisonette is £50 less; a house of three storeys or more is £50 more. One survey covers every service on the same job.
Plus the Planning Portal's own charge if the application goes through the Portal: up to £149 including VAT, depending on the application type, and some types have none (source).
Get this fee in writingor email hello@fixedfeeplans.co.uk
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Frequently asked questions
Is there a percentage limit in Hertsmere? No. SADM26 sets no figure; it compares floor space, volume, site coverage, height and use, and asks whether the addition is subordinate.
Does my patio or driveway matter? It can. Site coverage of building and hardstanding is one of the five comparisons, so large new hard surfaces count against the scheme.
Does a basement count? It may. The policy footnote says the original, extension or replacement building may include a basement level.
My house is in Shenley village. Is the Green Belt test lifted? No, if it is inside the village envelope: SADM23 says those areas are part of the Green Belt. Porters Park is a separate case; check the Policies Map.
