Methodology & Assumptions | Mortgage Payment Scenario Builder

Purpose & Intended Use

The Mortgage Payment Scenario Builder is designed for payment explanation, scenario comparison, and client-facing presentation. Its role is to help a user understand how a change in one or more assumptions can affect a modeled monthly housing payment, loan amount, loan-to-value ratio, amortization profile, and total modeled interest.

The tool is intentionally positioned as an explain-first model, not a lender production system. It does not attempt to replicate every rule, fee, reserve method, servicing convention, disclosure workflow, or program overlay that may appear in an actual mortgage file.

The primary use case is side-by-side scenario framing. Scenario A can be used as a baseline, and Scenario B can be used to isolate the effect of a changed interest rate, down payment, term, property tax assumption, insurance estimate, or HOA input.

What the Model Includes

The builder combines a fixed-rate principal-and-interest amortization model with monthly planning estimates for common non-loan housing cost components.

Home price: Used as the starting purchase-price assumption for the scenario.
Down payment: Entered as either a dollar amount or a percent of price and used to derive the modeled loan amount.
Interest rate: Used as the note-rate assumption for the amortization model.
Loan term: Entered in years and converted to months for amortization.
Property taxes: Included as a monthly planning estimate using either annual tax input ÷ 12 or a home-price tax-rate assumption ÷ 12.
Insurance: Included as an annual homeowners insurance estimate divided by 12.
HOA: Included as a direct monthly budgeting input for all-in housing cost visibility.
Monthly payment outputs: Includes modeled principal and interest plus monthly estimates for taxes, insurance, and HOA when entered.
Amortization outputs: Includes payment progression, beginning balance, principal portion, interest portion, cumulative interest, and remaining balance over time for the loan portion only.

Core Loan Structure

The model first derives the loan amount by subtracting the entered down payment from the entered home price. It then applies a standard fixed-rate amortization structure to that loan amount using the entered rate and term.

Loan amount is defined as: home price minus down payment.

LTV is defined as: loan amount divided by home price, expressed as a percentage.

P&I refers only to the modeled loan payment: principal and interest. Property taxes, homeowners insurance, and HOA are treated as separate payment components for total-cost planning and are not amortized into the loan balance.

Rate Conversion & Payment Convention

This model uses a simplified monthly-rate conversion convention: APR ÷ 12. That monthly rate is then used in the standard fixed-payment amortization formula for a fully amortizing loan across the entered term.

This convention is intentionally transparent and internally consistent for scenario comparison. It allows two scenarios to be measured against each other using the same rules, which is the core purpose of the interface.

Real-world lender systems may use different terminology, disclosure treatment, rounding, or timing rules. Depending on the workflow, a real file may involve additional conventions not represented here, including fee treatment, escrow reserves, mortgage insurance rules, payment timing rules, and servicing-specific adjustments.

Amortization Method

The amortization schedule is modeled as a fixed-rate, level-payment schedule. For each payment period, the interest portion is calculated from the remaining loan balance, and the rest of the modeled payment is allocated to principal reduction.

Because interest is calculated from the remaining balance, earlier payments in the schedule generally show a larger interest component and a smaller principal component. As the balance declines, the interest share tends to decline and the principal share tends to increase.

The month inspector and timeline charts are presentation layers on top of this amortization structure. They are meant to make the changing payment composition easier to understand during a live explanation or scenario review.

Taxes, Insurance, and HOA Treatment

Property taxes, homeowners insurance, and HOA are presented as smooth monthly planning values. This is done to create a clearer all-in monthly payment view for comparison and presentation.

Property taxes can be entered as either:

annual property taxes, divided by 12, or
property tax rate, applied to home price and then divided by 12.

Homeowners insurance is treated as an annual amount divided by 12. HOA is treated as a direct monthly planning input.

These amounts are useful for total-payment visibility, but they do not represent a full escrow administration model. Actual collection methods, reserve requirements, reassessments, premium changes, shortage cures, and lender or servicer rules can materially affect real-world payment behavior.

Scenario Comparison Framework

The tool is built around a two-scenario framework: Scenario A and Scenario B. This allows users to hold one scenario constant while adjusting selected assumptions in the other.

The most defensible use pattern is to change one variable at a time whenever possible. That makes it easier to isolate what is actually driving the change in:

monthly principal and interest,
total monthly housing payment,
loan amount,
loan-to-value ratio, and
total modeled interest over the term.

When multiple assumptions change at once, the comparison can still be useful, but the explanation becomes broader because more than one variable is contributing to the difference.

Charts, Month Inspector, and Summary Outputs

The balance chart, payment composition chart, month inspector, and scenario comparison summary are intended to improve interpretability, not to replace a formal mortgage disclosure package.

The balance chart visualizes remaining balance over time for the loan portion. The payment composition chart visualizes how principal and interest interact across the schedule. The month inspector provides a point-in-time view of the selected month’s beginning balance, payment composition, cumulative interest, and remaining term.

The scenario impact summary is a structured explanation layer that helps users understand which outputs changed and why those changes matter in the context of side-by-side scenario review.

Exports & Presentation Materials

CSV and PDF exports are generated to support explanation, documentation, and presentation workflows. Their purpose is to make the scenario outputs portable and easier to share in a client-facing context.

Exported materials should be interpreted as presentation outputs derived from the same model logic shown in the interface. They are not lender disclosures, underwriting decisions, lock confirmations, fee worksheets, or final payment commitments.

The presence of borrower, property, or prepared-by fields supports presentation context only. Those fields do not change the model’s underlying arithmetic.

Known Exclusions & Important Limits

This model intentionally excludes a number of items that may affect actual mortgage cost, qualification, or disclosure results.

PMI or MIP unless accounted for outside the tool,
lender fees, discount points, or credits,
closing costs and prepaid items,
escrow reserve calculations, shortages, or cushions,
utilities, maintenance, or non-HOA property expenses,
program-specific overlays and underwriting adjustments,
servicing timing rules and day-count conventions,
ARM logic or variable-rate payment structures,
tax deductibility treatment, and
personalized affordability, suitability, or advice outputs.

For that reason, the tool should be understood as a structured educational model rather than a final source of truth for a specific live transaction.

Data Handling & Privacy Positioning

The interface is designed to run locally in the browser with no required account workflow and no built-in data storage requirement for basic use. That supports a cleaner, lower-friction client presentation experience.

Users should still apply their own professional judgment about what information they enter into any presentation tool and whether any additional compliance or internal workflow standards apply to their organization.

Interpretation Standard

The correct way to read this tool is: “These outputs show what the model does under the entered assumptions.”

The incorrect way to read this tool is: “These outputs guarantee the exact final loan structure or exact final payment a lender will deliver.”

That distinction matters. The system is designed to support clarity, consistency, and confidence during mortgage explanation, while still remaining neutral about recommendations and transparent about model boundaries.