Feature spec
Goal: Send a clear invoice after finishing a job.
- Add a client, line items, a due date, and payment instructions.
- Preview the total before exporting a PDF.
- Keep an editable draft if export fails.
App Overview
Mortgage Calculator™
*** SALE NOW ON - 50% off ***
$1.99 · In-app purchases
PRICE
$1.99
In-app purchases
BUSINESS MODEL
Paid
No overall Grossing estimate available
RATING
★ 4.7
140 ratings · Top 15%
Evidence scope: No current overall US chart position is available. Category-chart positions shown in discovery are a separate scope. Revenue is a directional model estimate, not verified earnings.
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Build Kit
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A product spec, screen plans, and build steps for your coding agent.
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What's included17 files
A product spec, phased build plan, differentiation notes, and screen references.
- START_HERE
- AGENTS
- EVIDENCE
- SPEC
- BUILD_PLAN
- DIFFERENTIATION
- skills/
- screens/
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Illustrative example: an invoice app for independent contractors. Your kit is tailored to the app you choose; this is not its generated content.
Screen plan
Invoice editor: Client at the top, editable line items in the middle, total and preview action at the bottom.
States: Empty draft, validation errors beside each field, exporting, and a retry action that preserves the draft.
Differentiate and validate
Hypothesis: Contractors need faster repeat invoices more than more templates.
First experiment: Test duplicating a previous job with five contractors. Watch where they hesitate before expanding the feature set.
The full kit adds a phased build plan, evidence notes, tool prompts, and implementation guidance. It is a plan for your coding agent, not a finished app.
Usage: Mechanics are fair game; never reuse the original name, branding, assets, or verbatim copy.
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Free quick prompt For a smaller starting point
Quick prompt
Good fit for mobile-first iOS and Android prototypes with managed app scaffolding.
// RORK SYSTEM PROMPT
Rork plan preview for Mortgage Calculator™: map the core Finance workflow, choose the smallest differentiated feature set, define the data and monetization boundaries, then prototype the riskiest user journey first.
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Opportunities
AI-GENERATED OPPORTUNITY HYPOTHESES
These are generated suggestions, not verified review quotations. Check current App Store reviews and speak with users before treating a possible gap as a requirement.
Hypothesis 1
Include more adjustable variables for personalized results.
Assumption to check: Limited customization options for calculations
Validate with people who use this workflow before building.
Hypothesis 2
Add charts or graphs to visualize payment breakdown.
Assumption to check: No visual representation of amortization schedule
Validate with people who use this workflow before building.
Hypothesis 3
Offer a freemium model with essential features available for free.
Assumption to check: In-app purchase for basic features
Validate with people who use this workflow before building.
Hypothesis 4
Simplify onboarding with tutorial or guided setup.
Assumption to check: Complex UI for first-time users
Validate with people who use this workflow before building.
App evidence
Original App Store material. Ratings and screenshots describe the existing app, not proof of demand for your version.
App Store description
*** SALE NOW ON - 50% off *** *** Featured in the Sunday Times App List 2012 *** *** Featured as 'What's Hot' by Apple *** 'Mortgage Calculator' has one aim - To SAVE YOU MONEY on your mortgage! If you're buying a house, looking to re-finance, or simply want to make the most of your existing mortgage, then this is the perfect app for you. Try out various 'What-If' scenarios and quickly see how extra payments can save you a whole lot of time and money. You can enter any number of Monthly, Yearly or One Off extra payments, at any period during the loan. Then watch the total loan length and cost go down. You may be surprised by just how much! Easily visualise your cost savings in the simple and interactive charts. Just slide your finger along to see the costs for each year. 5-Stars - "...beautiful, simple graphs which make tricky decisions a whole lot easier." 5-Stars - "...really helped me to compare the different mortgage deals" *** Highlights *** -------------------- Specially designed for the US market: √ See effect of regular or one off Extra Payments √ Variable Rates support √ Calculate the 'True Cost', by including Tax etc √ Takes into account PMI √ Try out various 'What-If' scenarios √ Email a PDF or HTML of the loan details √ Support for ARM 3/1, 5/1, 7/1 mortgages √ Charts allow easy visualisation of the costs √ Make an informed choice by comparing loans √ Backup & Restore via Dropbox √ Passcode protection √ Elegant, simple and powerful user interface *** Quick Calculators *** ----------------------------- Super quick calculators for instant answers: √ How much can you afford to borrow? √ Monthly Payments √ Money Saving Effect of Extra Payments √ Cost of Moving House *** Analyse & Compare *** --------------------------------- Visualise mortgage costs in simple, easy to understand charts: √ Store details of multiple mortgages √ Easily see the costs for each year with a slide of the finger √ Try out various 'What-If' scenarios √ Extra payments can 'Reduce Term' or 'Reduce Payments' √ Include Tax, Insurance, PMI in the payments √ Support for 'Repayment' or 'Interest Only' √ Monthly breakdown of costs (Amortization) √ Compare up to 4 mortgages side by side √ Support for ARM 'Initial Rates' e.g. 4% for 3 years *** Voted #1 Property App - upack.com *** --------------------------------------------------- "The strongest arrow in its quiver is the way it handles hypothetical situations, letting you analyze all manner of mortgage-related queries, such as how much you can afford to borrow, and a variety of other what-if scenarios..." Note that multiple currencies are supported!
Written user reviews are not shown here. Check current App Store reviews >
Screenshots
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Build scope
Plan a focused first version with your coding agent. These are planning assumptions, not a delivery guarantee.
Define the first version
Choose one audience and one core workflow. Use the kit to agree on its screens, data, and acceptance criteria before building.
Set boundaries
Decide which secondary features, integrations, and platform support can wait. Your version does not need to reproduce everything in the original.
Check the hard parts
Validate external services, specialist technology, data access, and ongoing costs for your chosen scope.
SUGGESTED BUILD STACK
React Native
Expo
React Navigation
React Native Paper
@react-native-community/async-storage
Typescript
BUILD KIT DIFFICULTY
Intermediate
From the Build Kit assessment.
A reliable timeline needs an agreed scope and a technical check. Ask your agent to estimate the phases in BUILD_PLAN.md after that review.
