Model/CONTEXT.md

Ara

The Domna product for domestic retrofit modelling: ingests open-source EPC data, lets users correct or supersede it with their own surveys, and produces optimised retrofit packages for each property in a portfolio.

Language

Product

Ara: The Domna product. Latin for "the altar"; named under Domna's classical-naming convention. Covers both the modelling product and the backend that powers it. Avoid: ARA (acronym style), v2 backend, the new backend

Domna: The company. Roman name; sibling to Ara in the same naming convention.

Energy Performance Certificates

EPC: An Energy Performance Certificate — a government-issued document rating a dwelling's energy efficiency from A (best) to G (worst). Avoid: energy certificate, energy report

Certificate Number: The unique identifier assigned to an EPC by the government registry. Avoid: cert number, EPC ID

Registration Date: The date an EPC was lodged with the government register; used to identify the most recent certificate for a property. Avoid: assessment date, submission date

EPC Band: A single letter A–G representing a property's current or potential energy efficiency rating. Avoid: energy rating, EPC grade, EPC score

Schema Type: The versioned RdSAP or SAP schema that describes the structure of an EPC's raw data (e.g. RdSAP-Schema-21.0.1). Avoid: schema version, EPC format

Domestic Certificate: An EPC issued for a residential dwelling, as opposed to a commercial one. Avoid: residential EPC, home EPC

Properties and addresses

Property: The Ara domain aggregate representing a single dwelling under modelling: its identity, source data, enrichments, and modelling outputs. Avoid: dwelling, unit, home, asset

Properties: A first-class collection of Property objects; the unit of bulk operation in services. Avoid: property list, batch (used for SQS chunks)

UPRN: Unique Property Reference Number — the government-issued permanent identifier for a physical address in the UK. Avoid: property ID, address ID, code

Postcode: A UK postal code used to group nearby addresses; the primary search key for finding EPC records. Avoid: zip code, postal code

User Address: A free-text address string provided by a user or imported from a customer dataset, before any normalisation or matching. Avoid: user input, raw address, user_inputed_address

Comparable Properties: The reference cohort matched to a target Property by both geographic proximity (postcode prefix / UPRN range) and physical similarity (property type, built form, age band); used by the EPC Prediction Service for gap-filling and anomaly detection. Avoid: neighbours, similar properties, peer set

Source data

Site Notes: The full-coverage record produced by a Domna survey of a single Property; carries every EPC field the modelling pipeline requires, and when present supersedes the public EPC for that Property — except when the public EPC is newer. Avoid: energy assessment, site survey, field survey, Domna survey, Hestia survey

Landlord Overrides: Property data supplied by a landlord that may correct or supplement the public EPC for a single Property; triggers Rebaselining when applied; not applicable when Site Notes are present. Avoid: patches (deprecated), corrections, manual EPC, edits

Modelling

Effective EPC: The EpcPropertyData scored by the modelling pipeline for a single Property, derived from either Site Notes alone or the public EPC with Landlord Overrides applied; carries source-derived physical fields and originally recorded performance values, with model-rebaselined performance held separately in Baseline Performance. Avoid: modelling EPC, working EPC, resolved EPC, derived EPC

Rebaselining: Re-predicting a Property's SAP, carbon emissions, and heat demand via ML so the modelling pipeline scores it against the current SAP10 methodology. Triggered when either (a) the Effective EPC was lodged under a pre-SAP10 schema (sap_version < 10.0), so the recorded scores reflect a superseded methodology, or (b) Site Notes / Landlord Overrides changed the physical state of the Property (walls / heating / windows / etc.) so the lodged scores no longer reflect what's installed. Both triggers may fire together. Produces Effective Performance; Lodged Performance is preserved unchanged. Does not include kWh — that is always derived deterministically by EPC Energy Derivation. Avoid: re-scoring, re-prediction, performance recomputation, refresh (for cache-freshness)

Baseline Performance: A Property's current performance aggregate, holding both Lodged Performance and Effective Performance plus annual kWh / fuel split / bills derived from the Effective EPC. Persisted as one row; surfaced as one block in the UI. Avoid: baseline predictions, predicted baseline, rebaselined values

Lodged Performance: The SAP / EPC Band / carbon emissions / heat demand recorded on the public EPC (or the Site Notes' as-surveyed values when Site Notes are the source) — unmodified by modelling. The half of Baseline Performance that says "what the government register says about this Property". Avoid: original performance, raw EPC values, recorded baseline

Effective Performance: The SAP / EPC Band / carbon emissions / heat demand the modelling pipeline actually scored against — equal to Lodged Performance when no Rebaselining trigger fires, replaced by ML output when triggered. The half of Baseline Performance that says "what we modelled". Avoid: modelled performance, rebaselined performance (only correct when rebaselining ran), scored values

EPC Energy Derivation: The deterministic process that derives a Property's annual kWh, fuel split across heating, hot water, lighting, appliances and cooking, and bills from the Effective EPC — applying a UCL Correction for known EPC over/under-prediction and deducing fuel type from the SAP heating fields. No ML. Avoid: kWh prediction, baseline kWh, energy estimation

UCL Correction: The per-band linear correction (Few et al. 2023, Energy & Buildings 288 113024) applied to EPC-modelled total primary energy use intensity to align it with metered consumption. Calibrated against gas-heated, non-PV homes in England and Wales rated under SAP 2012; the current implementation extrapolates it to all properties (open question §15.14). Avoid: UCL adjustment, energy correction, metered correction

EPC Anomaly Flag: A per-field indicator that a Property's value for an EPC field differs significantly from Comparable Properties; advisory only — surfaces in the UI to prompt user review, does not block modelling. Avoid: outlier, mismatch, divergence flag

Reference data

Fuel Rates: The current per-fuel rate (pence/kWh) and standing charge used to compute a Property's bills; time-versioned and regional, refreshed from Ofgem's published caps via an ETL. The Smart Export Guarantee rate sits in the same set as electricity_export. Consumed by EPC Energy Derivation. Avoid: fuel prices (commodity prices, different concept), tariff, energy cost

Carbon Factors: The per-fuel CO2 emission factor (kgCO2e/kWh) used to compute a Property's carbon emissions; time-versioned, refreshed from Defra's annual publication. Consumed by EPC Energy Derivation. Avoid: emission factors (ambiguous), CO2 rates

Outputs

Scenario: A named portfolio-level retrofit plan, built by a user in the scenario-builder UI and persisted before any modelling fires; carries the overall goal (e.g. Increasing EPC), budget, exclusions, housing type, and an ordered list of Scenario Phases. The model is triggered against one or more Scenarios at once; each Scenario yields one Plan per Property. Avoid: project, batch, run-set

Scenario Phase: One ordered step inside a Scenario, carrying a measure-type allowlist (e.g. "loft insulation and walls in phase 1; ASHP in phase 2"), an optional phase budget, and an optional phase target. A single-phase Scenario is one Scenario Phase with all measure types allowed and the full budget on it — there is no special-case path. Avoid: scenario stage, scenario step, tranche

Scenario Snapshot: A frozen copy of a Scenario pinned at trigger time, keyed by (task, scenario); used by the modelling pipeline so mid-run edits to the live Scenario do not affect an in-flight job. Snapshots are read-only and may be garbage-collected after the task completes. Avoid: scenario version, frozen scenario, pinned scenario

Plan: The per-Property output of one Scenario's modelling run; carries an ordered list of Plan Phases matching the Scenario's Phase shape. A Property modelled against N Scenarios in one trigger ends up with N Plans. Avoid: recommendation set, output, result

Plan Phase: The per-Property output of one Scenario Phase: the Optimised Package selected for that phase, the ending state snapshot (the Property's SAP / kWh / bills after the package is applied), and any Rolled-over Options that flow as candidates into the next Plan Phase. Avoid: plan stage, plan step

Rolled-over Options: Recommendations generated but not selected by the Optimiser in a given Plan Phase, that remain eligible as candidates in subsequent Plan Phases. Exact roll-over rule (automatic vs user-marked) is under design. Avoid: deferred measures, leftover recommendations

Recommendation: A single proposed retrofit measure for a Property, with its cost, SAP impact, kWh savings, carbon savings, and parts list. Avoid: suggestion, option

Optimised Package: The subset of a Property's Recommendations selected by the Optimiser Service for installation, chosen to satisfy the Scenario's goal subject to budget. Avoid: selected measures, default measures, optimal solution, recommended bundle

Measure Type: The catalogue classification of a retrofit measure (e.g. solar_pv, loft_insulation, ashp); one or more Recommendations reference the same Measure Type with property-specific cost and impact. Avoid: measure (ambiguous), category

Address matching

Lexiscore: A similarity score in [0, 1] between a User Address and a candidate EPC address; combines token overlap and character-level similarity. Avoid: score, match score, similarity

Lexirank: Dense rank of candidates sorted by Lexiscore descending; rank 1 = best match. Avoid: rank, position

UPRN Candidate: An EPC Search Result that is a plausible match for a given User Address, before scoring decides the winner. Avoid: match candidate, result

Score Threshold: The minimum Lexiscore (currently 0.6) below which no match is returned even if a candidate exists. Avoid: minimum score, cutoff

Ambiguous Match: A matching outcome where two or more candidates share Lexirank 1, making it impossible to select a unique winner. Avoid: tie, draw, duplicate

Best Match: The single UPRN Candidate with Lexirank 1 that meets or exceeds the Score Threshold. Avoid: winner, top result

API and integration

EPC Search Result: A lightweight record returned by the government domestic search endpoint — address lines, postcode, UPRN, band, and certificate number, but not full certificate data. Avoid: search row, EPC row, result

EPC Property Data: The fully mapped domain object produced after fetching and parsing a complete EPC certificate; the schema the modelling pipeline operates against. Avoid: EPC data, certificate data, parsed EPC

Old EPC API: The retired government API (epc.opendatacommunities.org) using HTTP Basic auth; decommissioned 30 May 2026. Avoid: legacy API

New EPC API: The replacement government API (api.get-energy-performance-data.communities.gov.uk) using Bearer Token auth. Avoid: new API, current API

Bearer Token: The auth credential required by the New EPC API; stored in the EPC_AUTH_TOKEN environment variable. Avoid: API key, auth token, secret

Relationships

• A Property represents a single physical dwelling for modelling; identified by (portfolio_id, UPRN) or (portfolio_id, landlord_property_id).
• A Property has zero or more EPCs across time, exactly one Effective EPC, zero or one set of Site Notes, and zero or one set of Landlord Overrides.
• An EPC belongs to exactly one Property and has one Certificate Number.
• An EPC carries an EPC Band and is identifiable by its Registration Date; the most recent one is the current.
• A UPRN identifies a physical dwelling permanently; it does not change when the property changes owner — but each portfolio gets its own Property keyed against it.
• When a Property has both Site Notes and a public EPC, the newer of the two derives the Effective EPC. Landlord Overrides apply only when the EPC is the source — never when Site Notes are.
• A Property's Baseline Performance holds two halves: Lodged Performance (the gov register's SAP / band / carbon / heat) and Effective Performance (what the modelling pipeline scored against). The two are equal unless Rebaselining fires.
• Rebaselining produces Effective Performance by ML re-prediction when either (a) the Effective EPC was lodged under a pre-SAP10 schema, or (b) the Effective EPC's physical state diverges from the lodged EPC. Lodged Performance is never overwritten.
• EPC Energy Derivation contributes the annual kWh, fuel split, and bills on every Property unconditionally, reading current Fuel Rates and Carbon Factors from their respective repos.
• The EPC Prediction Service uses Comparable Properties for both gap-filling and producing EPC Anomaly Flags.
• A Scenario carries one or more ordered Scenario Phases. Triggering the model against N Scenarios produces N Plans per Property; each Plan carries an ordered list of Plan Phases matching the Scenario's shape.
• Each Plan Phase holds its Optimised Package, the ending state snapshot, and any Rolled-over Options that flow as candidates into the next Plan Phase. A single-phase Scenario is one Scenario Phase with all measure types allowed; the same machinery handles it.
• A Scenario Snapshot is pinned at trigger time per (task, scenario) so mid-run edits to the live Scenario do not affect an in-flight modelling job.
• A Recommendation references one Measure Type and carries property-specific cost and impact.
• Address Matching uses a User Address and Postcode to find a UPRN by scoring UPRN Candidates from an EPC search. A Lexirank of 1 with no Ambiguous Match and a Lexiscore ≥ the Score Threshold produces a Best Match.

Example dialogue

Dev: "A landlord uploads a corrected boiler for one of their properties. What happens?"

Domain expert: "That's a Landlord Override on the heating fields. Save it against the Property. The Effective EPC has changed, so Rebaselining runs to re-predict SAP / carbon / heat, and EPC Energy Derivation re-runs to update kWh / bills based on the new fuel deduction. With fresh Baseline Performance we regenerate Recommendations."

Dev: "What if the same Property also has Site Notes?"

Domain expert: "Site Notes supersede the public EPC, so Landlord Overrides don't apply. We model from the Site Notes version of the Effective EPC. If the public EPC is newer than the Site Notes, that's the one exception — we use the newer one."

Dev: "After modelling we end up with a list of measures. Which ones get installed?"

Domain expert: "The Optimiser Service picks the Optimised Package — a subset of Recommendations that hits the Scenario goal within budget. The rest stay in the Plan as alternatives the user can swap in."

Dev: "I'm looking at a property where the EPC says cavity walls but every other house on the street has solid. Is that a bug?"

Domain expert: "That's an EPC Anomaly Flag. We compute it against the Comparable Properties for that postcode. It's advisory — the UI surfaces it and the landlord can apply a Landlord Override if it's wrong."

Dev: "The property card shows two SAP scores side by side. Why?"

Domain expert: "Those are Lodged Performance and Effective Performance. Lodged is what the gov register says — the EPC was rated under SAP 2012. Effective is what we scored against — we ran Rebaselining to predict the SAP10-equivalent rating because the methodology changed. Both stay on the Baseline Performance so users can see what's on record and what we're modelling against."

Dev: "A landlord wants a 3-year retrofit plan — fabric work this year, heat pump next, solar after. How do we model that?"

Domain expert: "Three Scenario Phases in one Scenario. Phase 1 allows fabric measures with this year's budget, phase 2 allows the heat pump with next year's budget, phase 3 allows solar. When we model, the Optimiser Service runs per phase against the rolling state — the heat pump is scored against the post-insulation property, not the original one. Each Plan Phase captures the Optimised Package plus the ending SAP / bills, and any Rolled-over Options that didn't make this phase's budget become candidates next phase."

Flagged ambiguities

• "property" was historically warned against in favour of "dwelling"; that has been inverted. Property is now canonical for the Ara domain aggregate. Legacy code still uses "dwelling" in places — treat as alias.
• "energy assessment" in the existing codebase (energy_assessment_functions, energy_assessments_by_uprn) refers to what is now canonically called Site Notes. New code uses Site Notes.
• "patch" / patch_epc in the existing codebase has been merged into Landlord Overrides; the original concept is deprecated.
• "already_installed measures" in the existing codebase is likely subsumed by Landlord Overrides ("we have a heat pump now" → override the heating fields). Final call deferred to implementation.
• "address" appears as both the raw User Address (free-text) and a structured field on an EPC Search Result (normalised lines). Always qualify: "user address" vs "EPC address" or "address line 1".
• "score" is used for AddressMatch.score() output, the lexiscore column, and informally. Prefer Lexiscore in domain discussions; reserve "score" for method-level code comments.
• "user_inputed_address" in backend/address2UPRN/main.py is a misspelling and a synonym for User Address — the canonical term. New code should use user_address.
• "EPC" is overloaded as both the document and the rating band letter. Use EPC for the document, EPC Band for the letter.
• "re-scoring" has two meanings in the codebase — Rebaselining (re-predicting baseline performance after an EPC change) and post-optimisation measure re-prediction. Prefer Rebaselining for the former; for the latter, the Optimiser Service step does its own scoring without a special name.
• "phase" appears in two unrelated contexts: as cut-over timeline language in the PRD ("Phase 0 — Status quo", "Phase 1 — Forced cut-over") and as a domain concept in Scenario Phase / Plan Phase. Only the latter is a glossary term; cut-over phases are project-management vocabulary that does not enter code.
• "stale" appears in two senses: cache-freshness ("a Repo record is stale and the orchestrator should refetch") — a legitimate operational concept; and as loose shorthand for the EPC's recorded cost fields being unusable. The cost fields are not stale — they are pinned to the inspection-date fuel rates by design. Use "pinned to inspection date" or "pre-SAP10 schema" (whichever applies) instead.