# Valuation Uplift is percentage-primary

The Modelling rebuild needs a financial-uplift output (the increase in a Property's
market value from a retrofit). The legacy model (`backend/ml_models/Valuation.py`) is
**value-primary**: it starts from a current market value and returns absolute pounds.
But that current value — a **Property Valuation** — is sourced from a customer upload
(or a ~93-entry hardcoded demo stub) and is **absent for the overwhelming majority of
Properties**, including every property in an EPC-only scale corpus. A value-primary
model therefore produces nothing for almost all inputs.

We model **Valuation Uplift** as **percentage-primary** instead: the uplift is computed
purely from the **EPC Band** jump (current → target) and is always returned as a
percentage; the absolute £ form (`lower/upper/average_value`, `post_retrofit_value`) is
derived **only when a Property Valuation is supplied**, otherwise left `None`. This means
every Plan gets an inspectable uplift even with no market value, and it cleanly separates
the two concepts the word "valuation" was blurring — the externally-sourced **Property
Valuation** (a Baseline attribute) from the plan-conditional **Valuation Uplift** (a Plan
output). The domain function lives in `domain/modelling/valuation.py` (Modelling is the
consumer that knows the target band; relocatable to a neutral package later, as
`domain/billing/` was, if Baseline takes ownership of Property Valuation).

## Consequences

- The percentage uplift compounds the legacy's four hardcoded broker tables
  (MoneySupermarket, Lloyds, Knight Frank, Rightmove), taking min/max/average across the
  sources that cover the band step. These 2022-era figures are ported verbatim as
  committed reference data; they are a provenance snapshot, not a live source.
- The **2× ROI cap** (uplift ≤ twice the retrofit cost) is a £ comparison, so it can only
  bite once a Property Valuation supplies the £ form; the bare percentages are uncapped.
- The model is a pure function of the before/after **EPC Band** — it does **not** use the
  continuous SAP score, so it needs no precision work beyond the band the Plan already
  computes.

## Deferred (not in this phase)

- **Property Valuation sourcing** — the upload-CSV ingestion slice, the Property field +
  persisted column, and the decision to retire or keep the demo `UPRN_VALUE_LOOKUP` stub.
  Where it persists (Baseline/performance table vs. a separate valuation table) is open.
- **Per-measure `property_valuation_increase`** and **`rental_yield_increase`** — the
  legacy path never populated either; uplift is a plan-level figure for now.