DETAILED FORMULA
A jobs & population‑led, corridor‑first, rail‑weighted method that sets housing need at the SDS (strategic planning authority)/rail‑corridor scale, then distributes it to stations and LPAs in proportion to public‑transport connectivity and deliverable, design‑led capacity.
This method is an alternative to the standard method for authorities choosing to adopt the Sustainable Growth Housing Method (SGHM). It sets out the calculation of a minimum annual housing need at corridor/Travel to Work Area (TTWA) level, the distribution to local planning authorities (LPAs), and the rules for allocation and monitoring. SGHM should be read alongside policies for sustainable locations (TR3), densities (L3) and plan‑making/requirement setting (HO1–HO3).
Step 1 – Define geography and units of allocation
Identify the functional geography: TTWAs and/or rail corridors covering one or more LPAs (or the Spatial Development Strategy area). Define station walkable pedsheds (typically 800–1,000 m) to act as delivery units for distribution.
Data sources: ONS TTWAs; national station and timetable datasets (DfT/GBR); adopted or emerging Spatial Development Strategy (SDS) geography.
Step 2 – Establish the corridor baseline (jobs, population and migration)
For each corridor/TTWA c, compute a household baseline driven by jobs, population and migration:
Baseline_HH_c = HH_trend_c + γ · (ΔJobs_c · E2H) + δ · NetMigration_c
- HH_trend: household growth from ONS household projections (most recent base), using the migration‑variant where advised;
- ΔJobs: net employment growth using ONS BRES trends plus evidenced local pipeline;
- E2H: jobs to households elasticity (policy‑set, calibrated to commuting/participation);
- γ, δ: national parameters (optionally tuned at SDS level).
Data sources: ONS household projections/SNPP; ONS BRES; agreed local employment pipeline evidence.
Step 3 – Apply a bounded affordability calibration (±10%)
Apply a capped calibration to reflect market signals without over‑ or under‑calling need:
AdjBaseline_HH_c = Baseline_HH_c · (1 + Calib_c), where Calib_c = clamp(f(Affordability_c), −0.10, +0.10)
The exact form of f is set nationally; the ±10% cap is mandatory. Use ONS median workplace‑based affordability ratios (five‑year mean).
Step 4 – Compute the Rail Accessibility Index (RAI) for each station pedshed
For every pedshed i, compute an index reflecting present (Gear‑1) or committed (Gear‑2) public‑transport quality:
RAI_i = w1·Freq_i + w2·EvenInterval_i + w3·TimeToHub_i + w4·Access&Interchange_i
- Frequency: daytime trains/trams per hour;
- Even‑interval: clockface regularity;
- TimeToHub: generalised time to key hubs;
- Access & Interchange: step‑free access, safe walking/cycling links, feeder bus/tram/BRT integration.
Normalise RAI around 1.0 (e.g., 0.8–1.3) and set weight_i = RAI_i / mean(RAI). Weight parameters w1…w4 are national defaults (locally adjustable within a published range).
Data sources: DfT/GBR timetable and station datasets; local access/interchange audits; Local Transport Plans (LTP) and LCWIP.
Definition used by SGHM (service‑led): a High‑Connectivity station provides (or is committed to provide) at least 4 trains/trams per hour overall (≥2 – ideally 4+ - each way) on an even‑interval pattern with step‑free, integrated interchange; stations qualify on adoption of a committed, funded corridor plan (Gear‑2).
Step 5 – Establish deliverable capacity at policy‑led densities
For each pedshed i, estimate capacity consistent with national density minima and local design codes:
Cap_i = min(Land_i · NetDevShare · Density_i, InfraCap_i)
Density minima (L3): ≥40 dph within reasonable walking distance of a station; ≥50 dph where a station is high‑connectivity; higher ranges apply in urban cores through design codes.
Data sources: Site supply/constraints (plan evidence); utilities/water/education capacity; local design codes; Defra MAGIC constraints for sieving/environmental capacity.
Step 6 – Distribute corridor need to stations and LPAs; set LPA requirements
Distribute AdjBaseline_HH across pedsheds by weight_i × Cap_i, iterate if capacity binds, and roll up to LPA totals. The SDS (or joint planning body) publishes an Apportionment Statement with reasons, then HO2 requirement figures are set for each LPA.
Step 7 – Apply the national sequential test for allocations
LPAs allocate to meet their SGHM‑derived requirement using a national cascade:
Tier A – Station walkable neighbourhoods (≈800–1,000 m) of high‑connectivity/committed‑upgrade stations (apply L3 minima & codes);
Tier B – Other high‑RAI nodes with turn‑up‑and‑go bus/tram/BRT feeding rail (≤15‑minute headways) and coordinated interchange;
Tier C – Town/district centres consistent with L3 and TR3;
Tier D – Edge‑of‑settlement sites only if need remains;
Tier E – Other locations in exceptional cases with binding accessibility upgrades.
Step 8 – Two‑gear operation and monitoring
Gear‑1 applies where service levels are as‑is;
Gear‑2 applies on adoption of a Corridor Metroisation Plan (GBR/Mayoral agreement, funding and milestones), after which affected stations’ RAI is recalculated and their apportionment increases at the next review. Monitor a Metroisation Accessibility Metric (tph + interval + access/interchange) corridor‑wide.
3.1 Data sources (authoritative and update rhythm)
ONS population & household projections (latest base; use migration‑variant where advised) — annual/periodic.
ONS BRES workplace employment (annual), plus evidenced local pipeline.
DfT/GBR timetable and station datasets; local audits of access/interchange; Connectivity Tool evidence under TR3.
Defra MAGIC and local environmental/heritage constraints for capacity sieving.
3.2 Parameters (national defaults)
Jobs to households elasticity (E2H) and weights w1…w4 for RAI published nationally with allowed local bands; affordability calibration capped at ±10%; pedshed radius 800–1,000 m; density minima per L3.
3.3 SDS level (how to apply)
Run Steps 1–6 to generate corridor totals and LPA requirements; publish the Apportionment Statement with methodology, inputs and reasons.
• Where capacity in Tiers A/B is insufficient to meet the corridor requirement, plan‑makers should first pursue design‑led intensification and/or commit a Corridor Metroisation Plan to unlock additional capacity (Gear‑2) at the next SDS review. Only where a justified shortfall remains after these measures should residual need be distributed to lower tiers in the sequential cascade, having regard to environmental, heritage, flood risk and (where applicable) Green Belt policies.
3.4 LPA level (how to apply)
Allocate to meet the SGHM requirement figure following Step 7 sequential test and L3 minimum densities (use local design codes for higher ranges); evidence TR3 accessibility and PM8 use of standardised tools.
Where relevant (e.g., Green Belt releases), apply Draft NPPF policy requirements in GB3–GB8 in parallel; SGHM does not disapply Green Belt policy.
3.5 Monitoring and decision‑making interfaces
Five‑year housing land supply (5YHLS): identify and update a 5‑year supply of deliverable sites against the adopted SGHM requirement, with buffer as per Annex D rules.
Housing Delivery Test (HDT): monitor delivery against the adopted SGHM‑based requirement; apply the published HDT consequences and timings.
High‑Connectivity station (SGHM): ≥4 tph overall (≥2 – ideally 4+ - each direction) on an even‑interval pattern, with step‑free access and integrated interchange; or a station served under a formally adopted, funded Corridor Metroisation Plan with a reasonable prospect of meeting these criteria within the plan period (treated as Gear‑2 for apportionment).
Rail corridor (SGHM): A contiguous set of stations and service links (trunk + branches) that function as a single service family to one or more hubs, providing at least 2 tph per direction on the trunk in the daytime base timetable, used by the SDS as the strategic analysis unit for SGHM—defining the station walkable neighbourhoods to which housing need is distributed and from which LPA requirements are rolled up.
Pedshed: station walkable catchment used as a unit for distribution and capacity estimation; typically 800–1,000 m, adjusted for barriers/topography where evidenced.
A1. Definition recap. A rail corridor is the trunk plus any branches that function as a single service family to one or more hubs, providing minimum ≥2 tph per direction (ideally min 4 tph) on the trunk in the daytime base timetable.
A2. How to draw a corridor
Definition of corridor types used in SGHM
- National Corridor - An inter-city or regional trunk that runs into a major city-centre hub (or hubs), typically carrying mixed traffic (long-distance, regional, semi-fast and some stopping services). This corridor is usually heavy rail or metro on permanent way and sets the strategic spine into which other corridors connect.
- Metroisation Corridor - A predominantly local, turn-up-and-go stopping pattern over a 30-80 km geography, designed for even-interval, metro-like regularity. It may be heavy rail, metro/Underground, or tram/LRT on permanent way, often using short shuttle patterns and robust turnbacks to maintain regularity and reliable interchanges at the hub(s).
Corridor mode eligibility
- Any permanent-way mode (heavy rail, metro/Underground, tram/LRT) can form a corridor spine where it meets the frequency/regularity standards and connects into a hub for the ConnectedCity. Bus and BRT remain feeders (see feeder-node rules).
Note on spatial structuring
- Within a single SDS area, expect several ConnectedCities (federations of towns linked by rail), each anchored by one or more hubs and served by one or more Metroisation Corridors that interface with the National Corridor(s).
Step-by-step method
1) Choose anchor hub(s)
- National Corridor: select the primary city-centre hub(s) and any secondary regional interchanges that define the trunk.
- Metroisation Corridor: select sub-regional anchors (principal town centres or interchange nodes) that the local stopping pattern will connect at even intervals.
- Record which ConnectedCity (or ConnectedCities) each corridor primarily serves.
2) Build the service graph from the weekday base timetable
- National Corridor: group through-running services that share a trunk with >= 2 tph each direction (ideally 4+ tph), plus qualifying branches feeding that trunk.
- Metroisation Corridor: identify the local stopping pattern(s) capable of achieving even-interval headways (target <= 15-minute) on the core section—often via short shuttles or overlapping short workings that protect regularity and connections to the hub. Treat formally adopted, funded upgrades as Gear-2 for graph building.
3) Set corridor limits
- National Corridor: include stations on the shared trunk and those branches that maintain >= 2 tph to the hub (ideally 4+), terminating where the base pattern drops below the threshold or ceases to form a coherent family.
- Metroisation Corridor: include contiguous station pairs that the intended local pattern can serve at even-interval headways (<= 15-minute) in the base or committed timetable; the typical end-to-end extent is 30-80 km. Where the local pattern overlays a National Corridor, the Metroisation Corridor is drawn as the stopping-service layer on that spine (plus any qualifying branches).
4) Add station walkable neighbourhoods (pedsheds)
- Buffer every included station by approximately 800-1,000 m, adjusting for barriers/topography. These pedsheds are the delivery units for distribution and design-led capacity.
5) Recognise feeder nodes (non-permanent-way)
- Where bus or BRT provides <= 15-minute headways into either corridor type with coordinated interchange, treat the node as a high-RAI location for distribution purposes (but not as a rail station).
6) Publish corridor and ConnectedCity artefacts in the SDS evidence base
- For each corridor: map, classification (National vs Metroisation), mode (heavy rail/metro/tram-LRT), station/stop list, pedsheds, and the service schedules used (base and any committed Gear-2 plan).
- For the SDS area: the set of ConnectedCities, showing which corridors feed each one and how corridor overlays are treated in the analysis.
Operational notes (apply alongside A3 rules)
- Overlaps: a station may sit on both a National Corridor and a Metroisation Corridor; use a composite RAI (higher value or a published weighted blend) but do not double-count capacity—explain the treatment in the Apportionment Statement.
- Cross-boundary corridors: where a corridor crosses SDS boundaries, the lead SDS coordinates a shared run and publishes a joint statement.
- Service changes: when a Corridor Metroisation Plan is adopted/funded, re-weight affected stations and corridors as Gear-2 at the next review—raising RAI and apportionment around upgraded stations, with reasons and milestones recorded.
A3. Inclusion / exclusion rules
Minimum service thresholds (by corridor type)
- National Corridor (trunk plus qualifying branches): trunk sections must operate at >= 2 trains per hour (tph) per direction in the daytime base timetable; 4+ tph on the trunk is the strategic aspiration. Branches qualify if they feed the trunk at >= 2 tph to the hub.
- Metroisation Corridor (local stopping layer on permanent way: heavy rail, metro or tram/LRT): include contiguous station/stop pairs that can be operated to an even-interval (clock-face) pattern with turn-up-and-go headways on the core section (target <= 15 minutes). Where a Metroisation Corridor overlays a National Corridor, draw it explicitly as the stopping-service overlay on that spine (plus qualifying branches). Even-interval operation and frequency are core RAI components.
Modes
- Permanent-way modes (heavy rail, metro/Underground, tram/LRT) may constitute a corridor spine where they meet the threshold and connect to the hub(s). Non-permanent-way services (bus, BRT) do not constitute corridor spines under SGHM but can be designated as feeder nodes for distribution.
Feeder nodes (non-permanent-way)
- Treat bus/tram/BRT nodes that provide <= 15-minute headways into either corridor type with coordinated interchange as high-RAI locations for distribution (but not as permanent-way corridor stations/stops).
Overlays and overlaps
- Overlays: Where a Metroisation Corridor (heavy rail or tram/LRT) runs on the same physical alignment as a National Corridor, treat the Metroisation Corridor as the stopping-service overlay. Use the Metroisation service pattern when calculating station-level RAI for local distribution; avoid double-counting capacity.
- Overlaps: At junctions/interchanges a station/stop may sit on both corridor types; use a composite RAI (higher value or a published weighted blend), but do not double-count capacity. Explain the approach in the Apportionment Statement.
Cross-boundary corridors
- Where corridors cross SDS boundaries, the lead SDS coordinates a shared analytical run and publishes a joint statement (cooperation under PM10/PM11).
Service changes and Gear-2
- When a Corridor Metroisation Plan is formally adopted/funded, re-weight the affected corridor(s) and stations/stops as Gear-2 at the next review—raising RAI and apportionment around upgraded locations, with reasons and milestones recorded.
Exclusions
- Exclude stations/stops or segments that fail the minimum service standard for their corridor type in the base timetable, unless a committed, funded upgrade (Gear-2) will bring them within standard within the plan period.