Status: accepted

Accepted — merged with intent to implement.

RFC 0117: Extend the size contract to mixed-use component sizes and guard the per-size component metrics

Resolution

The UWMD maintainers accepted this extension on 2026-10-22 to resolve #490 and complete the size-contract coverage from #415 and RFC 0100. The decisions below extend the closed size table and per-size metric admission to mixed-use component sizes.

Summary

RFC 0100 made every field in the Format §5.5 size contract table refuse a negative or non-finite value (SIZE-01). It also made the per-size dictionary metrics unverifiable when the size they divide by is stated below zero (positive_size_denominator). The table covers property and rent_roll. It leaves out the mixed-use components slots, which carry their own sizes.

So components.retail.nra_sqft: -18000 validates clean, and the engine computes retail_price_psf = -333.33. This RFC adds the component size fields to the table and marks the eight component per-size metrics positive_size_denominator.

Change

1. Format §5.5 table gains one row for the components section, on every component slot (components.<class>. for each admissible class key):

   Field	Kind
   total_units, total_beds, total_sites	count
   nra_sqft, critical_it_load_kw	area / capacity

   Each field keeps its existing meaning in section-components.schema.json.

2. Dictionary. These eight mixed-use per-size entries take null_semantics: positive_size_denominator. Their formulas and round_to are unchanged:

   • multifamily_price_per_unit
   • retail_price_psf, office_price_psf, industrial_price_psf, self_storage_price_psf
   • student_housing_price_per_bed
   • data_center_price_per_kw
   • manufactured_housing_price_per_site

   Each divides by a component size that row 1 registers.

3. No other field joins the table (D2). The pass found these silent fields, which no Format section defines:

   • senior housing assisted_living_units, memory_care_units, licensed_beds;
   • manufactured-housing rent_roll.site_mix_summary[].count;
   • rent_roll.rentable_units, rent_roll.unit_mix[].occupied_units and .nrsf;
   • property.occupied_units;
   • parking_ratio_per_1000_sf, meeting_space_sqft.

   They are a producer's own fields. The closed table lists Format fields only, so a producer that wants them checked proposes them as Format fields first.

Validation impact

A record with a negative component size gains SIZE-01, and its component per-size metrics become unverifiable instead of computing a negative price. No record with lawful sizes changes outcome.

Conformance impact

New cases:

• a negative components.retail.nra_sqft (SIZE-01; retail_price_psf unverifiable);
• a fractional components.multifamily.total_units (SIZE-01);
• a zero component size, which is admissible but leaves the per-size metric unverifiable through the existing divide-by-zero rule.

No expected value of an existing case changes. The mixed-use fixtures state positive sizes.

Reference implementation

The Format table; spec/metrics/builtin.json and its generated mirror (eight null_semantics values); size-contract.ts and the Python uwmd_size_contract.py (the new rows, with the components.<class> path expansion); the metric verifier's denominator registry; the cases.

Alternatives considered

• Add the undefined fields too. The table would then name fields no Format section defines, which would make them Format fields by the back door. Rejected; see item 3.
• Refuse negative sizes only through the component schema's minimum. That is a schema-only rule with no validation code, and it breaks the single SIZE-01 story. Rejected.

Decisions

#	Question	Proposal	Why / alternative
D1	Which component fields	total_units, total_beds, total_sites (count); nra_sqft, critical_it_load_kw (area/capacity)	Every size field the component schema defines
D2	Fields no Format section defines	Out of scope; listed	The table is closed over Format fields (RFC 0100)
D3	Metrics guarded	The eight mixed-use per-size ids	Each divides by a newly registered size
D4	Versions	None allocated; prelaunch 4.x	RFC 0091 D6

Prior art

• RFC 0100 (size contract, SIZE-01, positive_size_denominator); RFC 0019 (components); issues #415 and #490.