Status: superseded

Superseded by a later RFC. Retained as design history.

RFC 0068: Manufactured-housing module — a community leased by the site, with park-owned homes as a second income line

Superseded by RFC 0080. Manufactured housing ships as a full builtin class; this module design is kept as design history.

Packaging superseded by RFC 0080. Official class content now ships as a full builtin. The recorded field, calculation and accepted decisions remain its design history.

Draft proposal. Drafted with AI assistance; reviewed by maintainers. An adopter's manufactured-housing plan is requirements evidence, not UWMD authority, and it is not verifiable in this repository.

On 2026-10-01 the maintainers selected decisions that narrow the first scope (see Decision status). Those directions do not accept the RFC or authorize implementation.

Summary

A manufactured-housing community (MHC) earns most of its income from pad rent: it leases a site to a resident who owns the home. Some sites also carry a park-owned home (POH), which earns home rent on top of the pad. No builtin class states either fact.

This draft proposes @uwmd/module-manufactured-housing, on RFC 0039's data-centre pattern. The module declares the custom class org.uwmd.manufactured_housing (fallback multifamily) and one required section, mhc_sites.

• Pad rent is one stated, source-backed scalar, weighted_pad_rent_monthly. This is the selected first-scope representation.
• Calculations are limited to MHC-specific and per-site metrics, all in ordinary existing calc grammar. None can divide by zero on a valid document.
• Generic and derived ratios are deferred. The debt metrics, the expense ratio and the POH share of rent are not in the first scope.
• Every required field is enforced by a runtime rule. The reference runtime does not validate documents against a module section's JSON Schema, so each required field has a deterministic rule.

No Format or Protocol change is proposed, and no version moves.

What exists today (re-verified on main at 51afadc)

• No MHC vocabulary. The builtin classes are closed for protocol 1.x (§2.2). None states sites, pad rent or a park-owned home.
• The module path. A module-declared class (RFC 0003, Protocol §X.2) is the designed path. RFC 0039 shipped one (@uwmd/module-data-center) with no spec change, confirmed that module formulas may read standard sections (its implementation note 1), and limited itself to vertical-specific calculations. It does not clone the generic debt pack.
• What a custom class does not get (Protocol §X.2.4). It has no builtin calc pack, no Excel layout and no §XIII size row. getAssetClassDefaults returns null for it, and CC-13 is not_applicable.
• Module runtime (module-runtime.ts):
  • calculations run in declaration order, each seeing earlier results as prior_results;
  • a validation rule is a safe expression in the §VIII.1 grammar that fires on false and is silent on null;
  • a rule that cannot evaluate is reported as MOD-RULE-ERROR, and a calculation that cannot evaluate as MOD-CALC-ERROR. Both are errors that name the rule or calculation;
  • a section's schema is only shape-checked when the manifest loads. Documents are not validated against it at runtime.
• Calc grammar limits that shape this design:
  • No collection iteration. sum(...nums) and avg(...nums) take explicit arguments; given an array, sum refuses with CALC-TYPE-001, and a numeric index such as site_types[0] does not parse.
  • No string literals. "2026-06-30" is CALC-PARSE-001, and there is no date predicate. A rule can test that a date is present, not that it is real.
  • if() is eager. It evaluates both branches, so a guard such as if(x > 0, 1 / x, null) still raises CALC-DIV-ZERO.
  • Type errors. An ordered comparison or arithmetic on a non-number is CALC-TYPE-001. == and != return booleans, including against null.
• Adopter evidence (unverified here). An adopter's planning record, dated 2026-09-29, treats the site as the underwriting unit, with POH home rent as a second stream.

Why the earliest calculation was unreachable

The original draft computed pad rent as Σ(site_types[].count × pad_rent_monthly) / total_sites, and two metrics depended on it. That needs element-wise products and a sum over a variable-length array, which the evaluator does not have. This RFC adds no collection iteration, aggregation primitive or second engine.

Nor could a rule check a counts-only site_types[] against total_sites. So the schedule is deferred, and pad rent is carried as one stated scalar.

Proposed first scope

Manifest

Field	Value
manifest_version	'1'
id	org.uwmd.manufactured_housing_module
requires_protocol	>=2.5.0 (the data-centre floor; the module uses no later feature)
requires_format	>=1.1
requires_tier	tier-3-calc-host
declares_asset_classes[0].id	org.uwmd.manufactured_housing
.display_name	Manufactured Housing Community
.fallback	multifamily (see Fallback)
.required_sections	['mhc_sites']
.optional_sections	none

Section mhc_sites (required)

Field	Requiredness	Type and unit	Meaning
total_sites	required	whole number ≥ 1	Every manufactured-home site in the community, occupied or vacant. The module's size denominator, since a custom class has no §XIII row. property gains no field.
occupied_sites	required	whole number, 0 ≤ n ≤ total_sites	Sites with a home in place under an in-place lease as of as_of_date.
park_owned_homes	required	whole number, 0 ≤ n ≤ total_sites	Homes the community owns on its own sites. 0 when it owns none.
weighted_pad_rent_monthly	required	currency per site per month, finite and ≥ 0	The authoritative pad-rent input, defined below.
poh_home_rent_monthly	conditional	currency per park-owned home per month, ≥ 0	Required when park_owned_homes > 0, and absent or null when it is 0. The average monthly home rent per park-owned home across all park_owned_homes: occupied homes at in-place home rent, vacant homes at asking home rent. It excludes the pad rent for that home's site, which weighted_pad_rent_monthly already counts.
as_of_date	required	YYYY-MM-DD	The date the counts and rents describe.

weighted_pad_rent_monthly is the selected first-scope representation. It is the site-count-weighted average monthly pad (lot) rent across all total_sites:

• occupied sites at their in-place pad rent;
• vacant sites at the asking pad rent the source or underwriter states.

Further rules:

• Scope. It is pad rent only, excluding park-owned-home rent, utility reimbursements, fees, and RV or transient income.
• Provenance. It is a stated, source-backed input (a rent roll, a T-12, or the underwriter's stated figure), carried with the section's _meta provenance. Core does not derive it, and no module claims to have computed it.
• Currency. Values are in the document's currency (RFC 0046).
• Range. It is finite and nonnegative, so zero is valid. JSON numbers are always finite, so the rule needs to check only presence and >= 0.

Calculations

Every formula is in the existing §VIII.1 grammar. Order is load-bearing: a row reads only stated fields and the rows above it.

id	Formula	Unit	round_to
physical_occupancy	mhc_sites.occupied_sites / mhc_sites.total_sites	%	4
annual_pad_rent_potential	mhc_sites.weighted_pad_rent_monthly * mhc_sites.total_sites * 12	$	2
poh_share	mhc_sites.park_owned_homes / mhc_sites.total_sites	%	4
annual_poh_home_rent_potential	mhc_sites.poh_home_rent_monthly * mhc_sites.park_owned_homes * 12	$	2
price_per_site	valuation.purchase_price / mhc_sites.total_sites	$	2
noi_per_site	noi_model.net_operating_income / mhc_sites.total_sites	$	2

Calculation audit

Metric	Denominator on a valid document	Null behaviour
Pad rent per site per month	none; it is the stated input weighted_pad_rent_monthly	—
physical_occupancy, poh_share	total_sites ≥ 1 (CC-MOD-MH-01)	null only if an input is absent, which a rule reports
annual_pad_rent_potential	none (a product)	Valid at zero when pad rent is 0
annual_poh_home_rent_potential	none (a product), so park_owned_homes: 0 cannot divide by zero	Valid at zero when park-owned homes exist and home rent is 0. null when there are no park-owned homes and therefore no home rent; poh_share is then 0
price_per_site, noi_per_site	total_sites ≥ 1	null when the standard-section figure is absent

poh_share is the first-scope measure of POH exposure. The POH share of rent is deferred (see Deferred).

No valid document can make a first-scope calculation divide by zero. A stated total_sites: 0 is invalid: CC-MOD-MH-01 reports it, and each per-site calculation also reports MOD-CALC-ERROR. AI performs none of this arithmetic; the evaluator does.

Validations (CC-MOD-MH-NN)

Every rule is an error, and there are no market or advisory thresholds. Each rule opens with mhc_sites == null ||, so a document missing the whole section reports MOD-SECTION-MISSING alone. Presence is tested with != null, which yields false rather than null on an absent field, so a missing required field is reported rather than ignored.

Code	Rule	Detects
CC-MOD-MH-01	mhc_sites == null || (mhc_sites.total_sites != null && mhc_sites.total_sites >= 1 && round(mhc_sites.total_sites, 0) == mhc_sites.total_sites)	total_sites missing, below 1, or not whole
CC-MOD-MH-02	mhc_sites == null || (mhc_sites.occupied_sites != null && mhc_sites.occupied_sites >= 0 && round(mhc_sites.occupied_sites, 0) == mhc_sites.occupied_sites && mhc_sites.occupied_sites <= mhc_sites.total_sites)	occupied_sites missing, negative, not whole, or above total_sites
CC-MOD-MH-03	mhc_sites == null || (mhc_sites.park_owned_homes != null && mhc_sites.park_owned_homes >= 0 && round(mhc_sites.park_owned_homes, 0) == mhc_sites.park_owned_homes && mhc_sites.park_owned_homes <= mhc_sites.total_sites)	park_owned_homes missing, negative, not whole, or above total_sites
CC-MOD-MH-04	mhc_sites == null || (mhc_sites.weighted_pad_rent_monthly != null && mhc_sites.weighted_pad_rent_monthly >= 0)	pad rent missing or negative (zero is valid)
CC-MOD-MH-05	mhc_sites == null || mhc_sites.as_of_date != null	as_of_date missing
CC-MOD-MH-06	mhc_sites == null || mhc_sites.park_owned_homes != 0 || mhc_sites.poh_home_rent_monthly == null	POH home rent stated when there are no park-owned homes
CC-MOD-MH-07	mhc_sites == null || mhc_sites.park_owned_homes == null || mhc_sites.park_owned_homes <= 0 || (mhc_sites.poh_home_rent_monthly != null && mhc_sites.poh_home_rent_monthly >= 0)	POH home rent missing or negative when park-owned homes exist

Non-numeric values. A string, boolean or object in a numeric field makes its rule fail to evaluate (CALC-TYPE-001). The reference runtime reports that deterministically as MOD-RULE-ERROR, naming the rule, together with a MOD-CALC-ERROR for each calculation that reads the field. The rule's own code cannot be emitted for this case, because the grammar has no type predicate.

as_of_date validity is not enforceable. CC-MOD-MH-05 detects a missing date. A malformed or impossible one, such as "2026-02-30", cannot be detected by the reference runtime: rule grammar has no string literals and no date predicate, and section schemas are not enforced. No calculation reads the date. Closing this gap needs a separate core capability, listed under Unresolved questions. This RFC does not rely on the manifest schema for it.

Verified on the real runtime. While drafting, these calculation and rule tables were run unchanged through evaluateModuleCalculations and validateAgainstModules on the current core build, using synthetic documents in a scratch harness that is not committed. 27 cases were run, and in each the issues matched the tables above:

• the manifest loads;
• valid documents with and without park-owned homes draw no issue;
• valid documents with zero pad rent, zero POH home rent with park-owned homes present, or both, draw no issue. The annual potentials are 0;
• valid documents with a zero loan_amount, a zero annual_debt_service, or zero EGI draw no issue;
• a missing section reports only MOD-SECTION-MISSING;
• each missing required field reports exactly its rule;
• each numeric violation reports its rule;
• non-numeric values report MOD-RULE-ERROR;
• a malformed as_of_date draws no issue, as stated above.

What the module does not do

• It does not derive pad rent from site types, and does not iterate.
• It does not compute generic debt metrics, the expense ratio, or the POH share of rent (see Deferred).
• It does not apply market thresholds, publish class defaults, or assume a value the document does not state.
• It does not model RV or transient sites.
• It does not reconcile its rent potentials with noi_model.income.gross_potential_rent. No rule ties them, because whether POH home rent sits in GPR or other income is a modelling choice with no evidence in the repository.

Fallback

multifamily remains the honest degraded reading. Under Protocol §X.2.2, a host without the module MAY render using the fallback's view models, MUST report the read as degraded and emit MOD-FALLBACK-001, and MUST NOT present it as a full read.

• What survives. The standard sections (property, ownership, noi_model, valuation, debt_structure, sources_uses, dcf) stay intact and render under residential income-property view models. The stated NOI, price, debt and returns read correctly. This RFC does not change those sections.
• What is lost:
  • the module's calculations and validations;
  • any labelled presentation of mhc_sites, which renders as a generic block.
• The total_units hazard. A host that also ran the multifamily pack (outside what §X.2.2 grants) would divide per-unit metrics by property.total_units. The example therefore leaves property.total_units unstated, so no mislabelled per-unit figure appears. RFC 0039 handled the same issue in its example.

Deferred

• Generic metrics for custom classes: loan_per_site, ltv, ltc, dscr, debt_yield and expense_ratio.

  • Copied into this module, a legitimately stated zero loan_amount, annual_debt_service or effective_gross_income would raise CALC-DIV-ZERO, which the module runtime reports as MOD-CALC-ERROR.
  • Absence and zero are different facts, and a document should not omit a true zero to avoid an error.
  • if() is eager, so no guarded variant is possible in today's grammar, and this RFC invents none.
  • The data-centre module already shows a vertical module computing only vertical-specific metrics.

  How custom classes should get generic metrics is a separate, shared design question. Builtin packs are unchanged.

• poh_rent_share (POH home rent as a share of pad plus home rent). Its denominator, annual_pad_rent_potential + annual_poh_home_rent_potential, is zero on a valid document when pad rent and home rent are both 0.

  • Requiring positive pad rent only to keep a derived ratio evaluable was rejected.
  • Because if() is eager, no conditional form is expressible today.
  • This RFC invents no engine workaround.
  • It can return when the calc contract has a suitable denominator or conditional mechanism, or when a use case justifies one.

  poh_share remains the first-scope POH-exposure metric.

• age_restricted. It is disclosure only, no calculation reads it, and the runtime would not enforce its type. It can return with a demonstrated consumer.

• The per-site-type rent schedule (site_types[]), including a counts-only inventory. No rule can reconcile it without collection iteration.

• Utilities detail (mhc_utilities). No calculation reads it, and it would duplicate noi_model.expenses.utilities and other-income figures.

• POH operating detail (occupancy, turnover, turn cost, reserves). No calculation reads it, and the reserve would compete with noi_model.expenses.replacement_reserves.

• RV and transient sites: no demonstrated consumer.

• Advisory thresholds: market judgements with no evidence.

• Module-published class defaults: the protocol has no mechanism.

Compatibility analysis

• Additive only. No Format or Protocol text changes and no version moves, following RFC 0039.
• Documents that don't load the module. A document that does not list the module in modules: is unaffected. A host without the module reads an MHC document as degraded multifamily (see Fallback).
• Standard sections. debt_structure, valuation, sources_uses and the other standard sections are unchanged and fully renderable. Stating zero debt values draws no module issue, because the module computes no debt metric.
• No builtin changes. No builtin calc pack, Excel layout or size-registry row changes.

Conformance impact (proposed)

A runtime suite under conformance/modules/runtime/, numbered after the data-centre scenarios at implementation. It would hold the example document plus fixtures for:

• Valid documents:
  • park-owned homes present (all six calculations pinned);
  • no park-owned homes (POH revenue null, poh_share 0);
  • zero pad rent, and zero POH home rent with park-owned homes present (annual potentials 0, no issue);
  • zero loan_amount and annual_debt_service stated (no module issue).
• Missing data:
  • the section missing (MOD-SECTION-MISSING only);
  • each required field missing (its rule only).
• Rule violations: each of CC-MOD-MH-01 to CC-MOD-MH-07.
• Non-numeric values: a non-numeric count (MOD-RULE-ERROR naming the rule).
• Fallback: the module not loaded, so the read is degraded with MOD-FALLBACK-001.

Each fixture pins its calculation results and validation codes. gen-conformance-cases would regenerate the runner cases, and verify-packages would gain the package.

Reference implementation (if accepted)

This draft authorizes none. If accepted, the implementation would land on a separate branch, as RFC 0039's did:

• packages/uwmd-module-manufactured-housing, laid out like packages/uwmd-module-data-center (manifest in src/index.ts, scripts/emit-manifest.mjs, tests);
• the example examples/Desert-Palms-MHC-Apache-Junction-AZ.uwx.md;
• the runtime fixtures above.

Adoption is the adopter's own follow-up.

Alternatives considered

• A builtin manufactured_housing class. The builtin set is closed for 1.x (§2.2), and the module path needs no spec change.
• A multifamily document stating sites as units. It loses the pad/home split and misstates the denominator.
• The original Σ over site_types[]. Unreachable without collection iteration.
• Both site_types[] and the scalar. Two truths that no rule can reconcile.
• Stating annual_pad_rent_potential instead of the monthly average. Superseded: the maintainers selected weighted_pad_rent_monthly.
• Copying the builtin debt metrics and expense ratio. A true zero would become an error, and no guard is expressible (see Deferred).
• Requiring positive pad rent so that poh_rent_share always evaluates. Rejected in review: a valid input is not restricted only to keep a derived ratio defined. poh_rent_share is deferred instead.
• Telling all-cash documents to omit zero debt values. It conflates absence with zero; rejected.
• Relying on the manifest JSON Schema for requiredness and types. The reference runtime does not enforce it; rules carry the contract instead.
• Publishing class defaults. The protocol has no mechanism; a core RFC would be separate.

Unresolved questions

1. as_of_date validity: a module-runtime capability gap. The reference runtime can require the date but cannot detect a malformed one. That needs a core capability, either:

   • a date-validity predicate in §VIII.3, or
   • runtime enforcement of module section schemas.

   Either is its own RFC. Until then this module enforces presence only.

2. Type errors. Non-numeric values surface as MOD-RULE-ERROR naming the rule, not under the rule's own code. A type predicate would need the same kind of core change. Whether MOD-RULE-ERROR is acceptable as the first-scope signal is a maintainer decision.

3. Generic metrics for custom classes: how a module-declared class should obtain debt metrics and the expense ratio without cloning formulas or refusing true zeros. This is a shared design question, outside this RFC.

4. Ratios with a possibly-zero denominator (poh_rent_share): whether the calc contract should gain a denominator or conditional mechanism, or whether a use case justifies one. Outside this RFC.

5. Acceptance of RFC 0068 and authorization to implement.

Decision status

RFC 0068 remains draft.

Item	Source	State
A custom class gets no builtin pack, layout, size row or defaults; the module supplies its own calculations	Protocol §X.2.4, RFC 0003	Existing rule.
The degraded fallback renders view models only and reports MOD-FALLBACK-001	Protocol §X.2.2	Existing rule.
No collection iteration; no string literals; if() is eager	Protocol §VIII.1, §VIII.3; evaluator	Existing behaviour.
Module rules fire on false and are silent on null; failures report MOD-RULE-ERROR / MOD-CALC-ERROR; section schemas are not enforced at runtime	module-runtime.ts, modules.ts	Existing behaviour.
Sites as the underwriting unit, with POH rent as a second stream	Adopter planning, 2026-09-29	Adopter evidence. Not verified here. Not authority.
weighted_pad_rent_monthly (site-weighted, all sites, vacant at stated asking rent; stated, not derived) is the first-scope pad-rent representation	Maintainer resolution, 2026-10-01	Selected for first scope.
age_restricted removed from first scope	Maintainer resolution, 2026-10-01	Deferred.
Generic debt calculations (loan_per_site, ltv, ltc, dscr, debt_yield) deferred; no recommendation to omit true zero values	Maintainer resolution, 2026-10-01	Deferred.
First-scope requiredness enforced by runtime rules, not the manifest schema	Maintainer resolution, 2026-10-01	Adopted in CC-MOD-MH-01..07.
expense_ratio deferred from first scope with the generic debt metrics	Maintainer resolution, 2026-10-01	Deferred.
poh_home_rent_monthly required (≥ 0) when park_owned_homes > 0, and absent or null when it is 0	Maintainer resolution, 2026-10-01	Decided for first scope (CC-MOD-MH-06, CC-MOD-MH-07).
weighted_pad_rent_monthly is finite and nonnegative (≥ 0), not strictly positive	Maintainer resolution, 2026-10-01	Decided for first scope (CC-MOD-MH-04).
poh_rent_share deferred from first scope; poh_share is the POH-exposure metric	Maintainer resolution, 2026-10-01	Deferred.
The class, fallback, section, six calculations and seven rules	This draft, within the accepted decisions above	Proposal.
as_of_date validity; type errors under MOD-RULE-ERROR	—	Unresolved. A module-runtime capability gap.
site_types[], utilities, POH operating detail, RV sites, thresholds, defaults, generic metrics for custom classes, poh_rent_share	—	Deferred.
Acceptance of RFC 0068 and implementation	—	Not decided. Not authorized.

Prior art

• RFC 0003 — module-declared asset classes and the fallback contract.
• RFC 0006 — the first module, and the rule semantics of firing on false and staying silent on null.
• RFC 0039 — the data-centre module, the template followed here, including module formulas over standard sections and the decision to compute only vertical-specific metrics.