Mini-project: Recreate a budget tracker like Monarch Money using free spreadsheets + hosted UI

Hook: Build a Monarch-like budget tracker without paid SaaS

If you're a developer or IT pro tired of recurring SaaS fees and vendor lock-in, you can prototype a modern, secure personal finance dashboard for near-zero cost. This mini-project uses Google Sheets or Airtable as a backend, a small static web UI for the front end, and free hosting/edge functions for secure API calls. No OAuth bank linking required — just reliable CSV/OFX imports, email/SMS parsing, and lightweight automation. In 2026, with static hosting and edge functions widely available on free tiers, this pattern is fast, inexpensive, and easy to iterate on.

Why this approach matters in 2026

By late 2025 and into 2026 we saw two trends that make this project practical for prototypes and side projects:

• Edge and serverless functions democratized secure APIs — small proxies let you keep secrets off the browser while staying on free tiers (Cloudflare Pages/Workers, Netlify Functions, Vercel Serverless). Keep an eye on vendor changes that affect free tiers and SLAs — see analysis on recent cloud vendor shifts for planning.
• Static-first UIs plus simple backends (Sheets, Airtable) provide a fast feedback loop for feature-driven prototyping — many teams now prototype core product flows before investing in full backend systems. If you prefer a WordPress micro-app or plugin route, check examples for micro-apps on WordPress.

What you'll build (high level)

• A data model in Google Sheets or Airtable to store accounts, transactions, categories, and budgets.
• A static web UI (HTML/CSS/JS) showing balances, transaction lists, category spend, and a simple budget view.
• Import options that avoid OAuth: CSV/OFX/QIF upload, automated Google Drive/Email imports, and optional (free) PSD2 connectors for EU users.
• A secure serverless proxy for writes and token-protected reads so you never expose API keys client-side.
• Free hosting on GitHub Pages / Cloudflare Pages / Netlify with serverless function hooks for sensitive operations.

Step 0 — Quick project plan (30–120 minutes for an MVP)

1. Define schema in Sheets or Airtable (accounts, transactions, categories, budgets).
2. Build a static UI with a transactions table, category filters, and a small chart area.
3. Implement CSV/OFX upload + mapping to transactions.
4. Add serverless proxy to protect API keys when writing to Airtable or when using Google Apps Script with an API key.
5. Deploy to free hosting and schedule an import job (Google Apps Script or Netlify scheduled function).

Step 1 — Data model: Sheets vs Airtable (choose your backend)

Google Sheets (fast to start)

• Pros: Instant familiarity, zero setup, flexible formulas, Google Apps Script for automation.
• Cons: Harder to scale if concurrency or relational logic grows; exposing the sheet publicly has privacy risks.
• Suggested tabs (sheets): accounts, transactions, categories, budgets.

transactions columns example: date, account_id, amount, currency, description, raw_import_id, category_id, rule_tag

Airtable (structured, API-friendly)

• Pros: Built-in record IDs, views, attachments, easy REST API, automations, and a nicer schema editor.
• Cons: Free plan rate limits and record caps; you must protect your API token (use a proxy).
• Suggested tables: Accounts, Transactions (linked to Accounts), Categories, Budgets.

Step 2 — Importing bank data without OAuth

Monarch and other consumer apps use OAuth-linked aggregators (Plaid, Tink). For a prototype you don't need that. Here are reliable, OAuth-less alternatives:

1) Manual file import (CSV/OFX/QIF)

• Most banks provide export of transactions as CSV, OFX, or QIF. Build a small UI component to accept file uploads and map columns to your data model.
• Automate mapping: sniff headers (date, amount, description) and provide a quick column-to-field mapper so users can confirm before import.
• For OFX/QIF, use small JS libraries (ofx-js or qif2json) to convert to JSON client-side.

2) Email parsing (automated imports)

• Many banks send transaction alerts via email. Use a free Mailgun/Mailjet mailbox + a serverless function to parse incoming email and extract transactions. Alternatively, use Gmail + Google Apps Script to scan specific labels and append rows to Sheets.
• This is great for ongoing imports without full account linking.

3) Google Drive / Google Pay exports workflow

• Some users already drop statements into a Drive folder. A Google Apps Script can watch a folder and parse dropped files to populate your sheet.

4) Lightweight PSD2 / Open Banking (EU) & free providers

• If you’re in the EU, services such as Nordigen offer a free tier for account and transaction access via PSD2. Many of these still use OAuth, but Nordigen's onboarding can be simpler for devs: you register an app and request access tokens; the flow is server-side-friendly.
• Be mindful of regional regulations and user consent — always store minimal data and log consent timestamps.

Tip: For a prototype, start with file and email imports — they’re immediate, privacy-friendly, and let you iterate on categorization before considering aggregator connections.

Step 3 — Categorization rules

Categorization is what makes a tracker feel like Monarch. Implement a rules engine that runs on import:

1. Exact merchant match (e.g., "STARBUCKS" -> Coffee).
2. Regex matches (handle variations like "AMZN Mktp" or "AMZN MKTP*EU").
3. Amount thresholds and negatives for refunds.
4. Manual overrides that persist to the row (category_id).

For Google Sheets, implement formulas + a simple Apps Script function that applies regex rules stored in a rules tab. For Airtable, use an automation (or a serverless proxy) to run rules and update category fields. Consider leveraging edge-driven personalization patterns to automate category suggestions for users.

Step 4 — Secure writes: serverless proxy patterns

Never embed Airtable API keys or Google Service credentials in client-side JS. Use one of these free proxies:

• Netlify Functions — environment variables to store API keys; supports scheduled builds and background functions.
• Cloudflare Workers / Pages Functions — fast edge locations, free tier, and KV storage for small state.
• Vercel Serverless — straightforward if you deploy your UI there; store credentials as Environment Variables.

Example Netlify function to write to Airtable (Node):

exports.handler = async (event) => {
  const AIRTABLE_KEY = process.env.AIRTABLE_KEY;
  const body = JSON.parse(event.body);
  const res = await fetch('https://api.airtable.com/v0/yourBase/Transactions', {
    method: 'POST',
    headers: { 'Authorization': `Bearer ${AIRTABLE_KEY}`, 'Content-Type': 'application/json' },
    body: JSON.stringify({ fields: body })
  });
  return { statusCode: res.status, body: await res.text() };
};

Deploy this as a function and call it from the UI. The environment variable keeps your key secret. For best practices on securing server-side components and secrets, review security playbooks for managed stacks.

Step 5 — Front-end UI: minimal, useful components

Keep the UI static and client-rendered using vanilla JS or a lightweight framework (htm/preact or Svelte) to stay fast on free hosting. Essential views:

• Accounts summary: current balances aggregated from transactions.
• Transactions list: filters (date range, account, category), inline edit for category override.
• Budget view: monthly budgets per category with progress bars.
• Import screen: upload files, map columns, preview results before importing.

Simple charting: use Chart.js or lightweight charting (Sparkline library) to show trends. For initial prototypes, pre-render small SVGs client-side.

Step 6 — Automation and scheduled imports

Automation reduces manual effort. Two common patterns:

• Google Apps Script — schedule a script to pull new files from Drive, parse them, and append to Sheets. Apps Script runs on Google's infrastructure (free quotas exist).
• Serverless scheduled function — Netlify Cron or Cloudflare Cron Triggers can run a function that polls an email parser or an aggregator and writes new transactions to Airtable.

Example: a Cloudflare Worker on a Cron Trigger fetches a Mailgun-saved JSON webhook and pushes parsed transactions to the Airtable proxy endpoint. Remember that outages or vendor changes can affect cron and function reliability — review cost and outage impact analyses when you rely on free tiers.

Step 7 — UX details that make it feel like a polished product

• Bulk categorize: allow selecting multiple transactions and applying a category or rule.
• Undo & audit trail: store raw_import_id and original text so you can revert mistakes.
• Local-first edits: let the user make temporary edits in the browser and sync them to the backend when the proxy confirms success.
• Export: provide CSV download of filtered results for backups.

Costs and limits — realistic expectations for 2026

Estimated resource use for a single-user prototype (monthly):

• Google Sheets / Airtable: free plans are generally sufficient for hundreds to a few thousand transactions. Airtable free limits apply (~1,200 records per base in older plans; check current limits).
• Hosting: GitHub Pages / Cloudflare Pages free for static sites; add free serverless functions on Netlify/Cloudflare. Monitor function invocation limits.
• Storage & parsing: email parsing services may charge for large volumes — use free tiers and route only relevant emails to the parser.

If you grow beyond free tiers, the upgrade path is simple: move the datastore from Sheets to a managed Postgres (Neon Hobby), keep Airtable for admin workflows, or switch to a paid aggregator if you need real-time account linking. For architectural options when monetizing data or running paid access, review approaches for architecting a paid-data marketplace.

Security & privacy checklist

• Never store raw bank logins. Use file/email imports or registered PSD2 providers with consent.
• Use serverless proxies to store API keys as environment variables — avoid exposing them in client JS. For concrete security steps with managed stacks, consult Mongoose security best practices.
• Encrypt backups or restrict sheet access if using Google Sheets; set the minimal share scope.
• Log data retention and provide a delete/export option for compliance and user trust.

Case study: 6-hour prototyping sprint

Here’s a realistic mini-case showing how a developer can iterate fast.

1. Hour 0–1: Create Google Sheet with Accounts/Transactions/Categories and sample rows; set up Apps Script skeleton to accept JSON POST and append rows.
2. Hour 1–2: Build static UI with fetch to a mock endpoint; show transactions and filter UI.
3. Hour 2–3: Add CSV upload & parsing on the client using PapaParse; map columns and preview rows.
4. Hour 3–4: Implement a Netlify function to accept parsed rows and write to Apps Script endpoint or Airtable; wire environment variables.
5. Hour 4–5: Add categorization rules and inline category edit; persist to backend.
6. Hour 5–6: Deploy static site to Cloudflare Pages or GitHub Pages, point domain, and schedule a simple Apps Script import job for new Drive uploads.

Result: a working personal finance tracker you can use and show in under a day. For production-grade security and aggregation, incrementally add paid connectors later. If you’re prototyping on a local edge device or experimenting with offline models, small local LLM setups on hobby hardware can complement local-first UX testing.

Advanced strategies & future-proofing (2026+)

• Modularize the proxy layer: make it simple to swap Airtable for Postgres or a managed timeseries DB later.
• Implement incremental sync tokens when using PSD2 providers — it avoids full re-imports and saves quota.
• Consider user-level encryption for sensitive notes and ensure GDPR/CCPA knobs are in place if you handle other users' data.
• Use observability on serverless functions (logs, usage alerts) so free-tier throttles don’t silently break imports — and plan for vendor changes by following cloud-vendor playbooks.

When to graduate from this prototype

Common signals that it's time to move off Sheets/Airtable:

• You need multi-user auth with role-based access and audit logs.
• Transaction volume approaches tens of thousands and you hit record caps or rate limits.
• You need true real-time bank connections and reconciliation — then evaluate paid aggregators. At that point, comparing CRMs and full document lifecycle tools can help pick a mature datastore and workflow.

Resources & libraries to accelerate development

• PapaParse — robust CSV parsing in the browser.
• ofx-js / qif2json — for converting OFX/QIF to JSON.
• Chart.js or Sparkline — simple client-side charts.
• Airtable API docs — quick REST endpoints for prototyping.
• Google Apps Script — for lightweight scheduled imports with Sheets.

Final practical checklist before you ship

1. Confirm no API keys are exposed in client code.
2. Test import pipeline with sample CSV, OFX, and email flows.
3. Implement a manual backup/export and a delete data path.
4. Monitor free-tier quotas weekly during initial rollout.

Conclusion & call-to-action

Recreating core features of a polished budget app like Monarch is entirely realistic as a low-cost mini-project in 2026. Use Google Sheets or Airtable to validate your UX and data flows, protect secrets with serverless proxies on free hosting, and prefer CSV/email/Drive imports to avoid the complexity and cost of OAuth-based aggregators until you need them. Start small — prioritize transaction imports, categorization rules, and a compact budget view; everything else is an iterative upgrade.

Ready to prototype? Clone our starter repo (static UI + Netlify function + sample sheet) and deploy on Cloudflare Pages in under 10 minutes. Share your fork or ask for a checklist for your specific bank’s CSV format — I’ll review and suggest mapping rules you can drop into your import pipeline.

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