Accredited official statistics

Local authority green belt: England 2023-24 - technical notes

Published 5 December 2024

Applies to England

1. Accredited Official Statistics Status

Accredited Official Statistics are called National Statistics in the Statistics and Registration Service Act 2007. These statistics have been independently reviewed by the regulator and found to comply with the standards of trustworthiness, quality and value in the Code of Practice for Statistics. The designation of these statistics as National Statistics was first confirmed in June 2012, following an assessment by the UK Statistics Authority.

The statistics last underwent a full assessment against the Code of Practice for Statistics in 2012.

More recently, the department underwent a compliance check by the Office for Statistics Regulation in Summer 2021. Its conclusions are set out in the letter from Mark Pont of OSR to Sandra Tudor (Head of Profession for Statistics).

Our statistical practice is regulated by the Office for Statistics Regulation (OSR). OSR sets the standards of trustworthiness, quality and value in the Code of Practice for Statistics that all producers of official statistics should adhere to. You are welcome to contact us directly with any comments about how we meet these standards. Alternatively, you can contact OSR by emailing [email protected] or via the OSR website.

2. Data collection and methods

Estimates of Green Belt land in individual local authorities in England are collected annually (as at 31 March) on the ‘Annual Green Belt’ (AGB) return via DELTA^{[footnote 1]},  the department’s electronic data collection system. Authorities were asked to confirm whether they agree with the Green Belt area estimate as calculated by the department as at 31 March 2023 and indicate whether they had changed the Green Belt boundary in 2023-24. If the Green Belt boundary had changed, or if they disagree with the estimated area as at 31 March 2023, authorities were contacted to obtain an explanation and to provide a revised geographic boundary file (shape file).

Robust estimates of the area of Green Belt land in England by local planning authority were first published in 1997. Between 1998 and 2002 there was no further publication on the area of Green Belt, but from 2003, Green Belt estimates were collated on an annual basis to monitor the department’s target on the area of Green Belt land in England. However, figures were not published for 2005 due to the extensive quality assurance required in that year.

The analysis carried out by the department calculates the area of the shape files supplied by local authorities and then aggregates them (together with previously provided boundaries for authorities who have not amended the Green Belt boundaries) to provide a national estimate of the area of Green Belt in England at a particular point in time. Any discrepancies are followed up with the authority and a corrected shape file obtained.

If there has been an actual change in Green Belt, rather than an improved measurement of its extent, this is followed up with the authority to determine the reason for the change. The statistics produced allow a comparison of Green Belt area between authorities and between different years.

The data collected have been chosen to provide useful and relevant information on the level and trends in the extent of the Green Belt to users while ensuring that the burden placed on local authorities of completing the return is not excessive. Only authorities who have changed the boundaries of their Green Belt, or who have corrected the boundaries, are asked to supply shape files.

As part of the 2022-23 statistical release revisions were made to the hectarage figures with the above methodology being applied to the hectarage figures from 1997 onwards. In addition, the area of land designated as Green Belt is now calculated using R software as opposed to QGIS software with area figures calculated in R being approximately 0.06% lower than those calculated in QGIS, at a national level this is equivalent to approximately 1,000 hectares. The largest revision due to the software change was a decrease of 0.08% for an individual authority.

3. Data quality

3.1 Data quality summary

Trends in the area of Green Belt can be reliably and easily assessed both nationally and at a local authority level. The high response rate (consistently over 90%) and quality assurance procedures followed result in accurate estimates of the Green Belt which should be suitable for many different uses. The flow diagram (Figure 1, below) sets out the key steps in producing and quality assuring the Green Belt statistical release.

The minimum expected response rate for the Green Belt return is 90%. The response rate has been consistently above this, as shown in the table below. The response rate for 2019-20 was relatively low due to the effects of the pandemic, due both to some local authorities being unable to submit data and the department focusing its efforts on maximising response rates for some other returns once the minimum expected response of 90% had been achieved. In 2023-24, the response rate was 100%.

Financial year	Response rate
2013-14	97%
2014-15	99%
2015-16	97%
2016-17	98%
2017-18	94%
2018-19	98%
2019-20	93%
2020-21	99%
2021-22	99%
2022-23	99%
2023-24	100%

3.2 Assessment of data quality

In 2015 the UK Statistics Authority (UKSA) published a regulatory standard for the quality assurance of administrative data. To assess the quality of the data provided for this release the department has followed that standard.

The standard is supported with an Administrative Data Quality Assurance Toolkit which provides useful guidance on the practices that can be adopted to assure the quality of the data they utilise.

The Green Belt Statistical release is produced by MHCLG based on data provided via its DELTA data collection system by 177 local authorities. An assessment of the level of risk based on the Quality Assurance Toolkit is as follows:

Risk/Profile Matrix Statistical Series	Administrative Source	Data Quality Concern	Public Interest	Matrix Classification
Green Belt Statistics	Local authorities’ individual planning systems	Low	Medium	Low Risk [A1]

The publication of Green Belt statistics can be considered as medium profile, as there is mainstream media interest, with moderate economic and/or political sensitivity.

The data quality concern is considered a low concern given that the data is collected from each local authority, via the data collection system DELTA which includes built in quality assurance checks. The data are then further quality assured by the responsible statisticians, who perform further detailed validation and checks, spotting and correcting any errors. Overall, the Green Belt statistics have been assessed as A1: Low Risk. A full outline of the statistical production process and quality assurance carried out is provided in the flow chart in Figure 1. Further details are also provided against each of the four areas outlined in the Quality Assurance Toolkit.

Operational context and administrative data collection

From 2007 to 2017, the department built and maintained its Green Belt dataset from local authority digital data supplied via the Interform return for local authorities, with a migration of this data collection to DELTA taking place for 2018 onwards. Authorities have also been required to provide the boundary of the Green Belt in their area on a suitable digital map to enable the publication of a shapefile for public use.

An England level dataset is created from the individual local authority boundaries, making sure that there is no overlap or duplication between different authorities. Trends in the area of Green Belt can be reliably and easily assessed both nationally and at a local authority level. All calculations are carried out using exact measurements, but published information is rounded to the nearest ten hectares. No imputation is carried out as authorities who fail to respond are assumed to have had no change in Green Belt, although statisticians check whether there is recent evidence online of a non-responding authority revising all, or a strategic part, of its Local Plan, given that Green Belt boundaries can be changed only through the Local Plan process. Authorities are given the chance to correct previously published estimates, whether due to actual changes that were not submitted in time, or because of more accurate Green Belt boundary information.

Communication with data supply partners

Authorities are supplied with comprehensive guidance to ensure that there is a common understanding of what information is to be supplied. Feedback on the return/statistical release is regularly sought from data providers and users, most recently regarding the move to DELTA data collection system, through a number of formats (Single Data List, the Central Local Information Partnership (CLIP) Planning Statistics Sub-group, and the data collection mailing list). The form is usually ‘live’ for six weeks. Should the minimum response rate of 90% not be met by the deadline, an extension is agreed and the Data Collection team will continue attempts to maximise the response rate until the new deadline. However, regular contact with local authorities as well as reminder emails throughout the time the form is ‘live’, result in relatively high response rates, as outlined in the section on response rates^{[footnote 2]}.

QA principles, standards and checks by data suppliers

Local authorities may measure their Green Belt using different software to that used by the department (e.g. QGIS as used by the department or other popular software such as ArcGIS or MapInfo). Depending on the mapping software used, a combination of methods may be implemented, which could result in different outcomes. Differences between the data provided could result from, for instance, different types of coordinate systems (geographic, i.e. a global or spherical coordinate system, or projected, i.e. on a Cartesian coordinate plane), or different scales of capture (e.g. OS Mastermap or OS landline).

As there are 177 different organisations supplying data to the department, it is not possible to fully understand and monitor each individual provider’s practical processes. To mitigate this lack of direct oversight, the department ensures additional effort is put into the QA checks carried out when the data is received. The department also reviews and updates guidance put out to local authorities each year, maintaining regular communication with providers throughout. All boundaries are checked to ensure that they match the area of the Green Belt as recorded by the local authority on the Interform/Delta return.

Producers’ QA investigations and documentation

Whilst local authorities are expected to carry out their own checks on the data submitted and are expected to specify the level of checks carried out in the form itself, discrepancies may arise due to use of different geographical software, or other different procedures used. Any such discrepancies are rare and are investigated with the local authority concerned to ensure that the calculated area of the Green Belt matches the boundary file supplied. The use of digital boundary files is much more cost effective and more accurate than the previous method of digitising paper maps. Given the increasing sophistication and accuracy of the software used to define and measure the Green Belt, estimates of its area should be very accurate and suitable for many different uses.

3.3 Imputation

Where a particular local authority has not submitted an AGB return for a particular year, departmental statisticians assume that the boundaries of the designated Green Belt land within its area did not change during the year, unless the authority is known to have revised all or a strategic part of its Local Plan in the recent past, when it is approached and asked directly whether its Green Belt boundary changed as a result during the relevant year. A judgement is then made on whether to include an estimated fgure for any change in the area of Green Belt land in the light of any response that is received.  Department statisticians assume that non-responding local authorities do not wish to revise any previously submitted information.

4. Revisions policy

This policy has been developed in accordance with the UK Statistics Authority Code of Practice for Statistics and the Ministry of Housing, Communities and Local Government Revisions Policy.

There are two types of revision that the policy covers:

Non-scheduled revisions

Where a substantial error has occurred as a result of the compilation, imputation or dissemination process, the statistical release, live tables and other accompanying releases will be updated with a correction notice as soon as is practical.

Scheduled revisions

Local authorities are asked to confirm the previously published area of their Green Belt when completing returns for each annual statistical release. If they do not agree with this figure, or they have changed the digital boundaries to more accurately represent the actual Green Belt boundaries, and the difference between the revised and previously published areas is more than ten hectares, they are asked to submit a revised shape file to the department.

Scale of revisions in recent years: the last seven scheduled revisions have resulted in changes to the overall area of the Green Belt in England:

• the area at 31 March 2014 was revised from 1,638,610 hectares to 1,633,390 hectares,

• the area at 31 March 2015 was revised from 1,636,620 hectares to 1,631,500 hectares,

• the area at 31 March 2016 was revised from 1,635,480 hectares to 1,630,470 hectares,

• the area at 31 March 2017 was revised from 1,634,700 hectares to 1,629,290 hectares.

• the area at 31 March 2018 was revised from 1,629,510 hectares to 1,623,180 hectares.

• the area at 31 March 2019 was revised from 1,621,150 hectares to 1,619,320 hectares.

• the area at 31 March 2020 was revised from 1,615,800 hectares to 1,619,750 hectares.

• the area at 31 March 2021 was revised from 1,614,000 hectares to 1,614,000 hectares (no net change).

• the area at 31 March 2022 was revised from 1,638,150 hectares to 1,637,560 hectares.

• the area at 31 March 2023 was revised from 1,638,420 hectares to 1,638,310 hectares.

Individual local authority revisions can, however, be more volatile.

5. Other information

5.1 Devolved administration statistics

Information and statistics on planning for the devolved administrations can be accessed at the following links:

• Scotland

• Wales

• Northern Ireland

In addition, data on Green Belts in Scotland are available.

The department is part of a cross-government working group, including devolved administrations and the Office of National Statistics, working to improve the trustworthiness quality and value of housing and planning statistics across the UK. This working group has published an action plan to make the planned improvements on house building and planning statistics clear and transparent to users. Details of this work and how you can provide feedback are available via the Government Statistical Service website.

5.2 User engagement

Users are encouraged to provide feedback on how these statistics are used and how well they meet user needs. Responses should be addressed to the contact given in the first page of the release.

See the Department’s engagement strategy to help it to meet the needs of statistics users.

The department routinely discusses planning and related statistics with representatives of local authorities in the Central Local Information Partnership (CLIP) Planning Statistics Sub-group and details of the Planning Statistics Sub-Group are available at [email protected].

1. The Annual Green Belt return was collected on DELTA for the first time in 2017-18; previously the data were collected on Interform. The type of data collected did not change, only the method by which the data are submitted. ↩

2. Should any significant changes to the form be carried out since the previous year, local authorities will be reminded that the form is due to go live four weeks in advance of the ‘go live’ date. ↩