How do you build a mortgage or affordability calculator?

The engineering work is almost entirely in the domain model, not the interface. We define the calculation — principal, term, rate, deposit, fees and the figures derived from them — as an explicit, single-source module, then expose it through an API so every surface asks the same engine the same question. The interface is deliberately the thin part, because a calculator that gives two different answers in two places is worse than no calculator at all.

How do you make sure financial calculations are correct?

By treating the arithmetic as the highest-risk code in the system and testing it accordingly. Boundary cases get the attention: very short and very long terms, unusually large deposits, rounding at each step, and the order in which fees are applied. We also keep the calculation rules separate from presentation, so a reviewer who understands mortgages but not web development can still read and check the logic.

Can a calculator handle changing rates and lending rules?

It should, and designing for that from the start is much cheaper than retrofitting it. We keep the assumptions that change — rates, thresholds, fee structures — as configuration rather than embedded constants, so updating them is a data change with an audit trail rather than a code release. That also means a historical result can be reproduced against the assumptions that were live when it was generated.

What stack suits a property or mortgage platform?

This one runs on Django with Django REST Framework and Celery, which suits the shape of the problem well: a strong domain model at the centre, an API so the rules are shared rather than duplicated, and a background worker for the heavier scenario runs. The front end is HTML and Sass. The stack matters less than keeping the calculation logic in one place that can be reviewed on its own.

HomeReal estate & PropTechCase study · P-015

Real estate & PropTech · Mortgage calculation platform

Mortega: mortgage calculation for property buyers.

A web platform that turns the arithmetic behind a property purchase into something a buyer can actually reason about. Built with Django, Django REST Framework, Celery, HTML and Sass, it models affordability, repayment and the cost of borrowing against a specific property, so the number a buyer sees is one they can act on rather than a teaser rate.

The problem

The challenge.

Buyers were assembling affordability estimates from calculators that each made different assumptions, then discovering the real number only once a lender was involved.
The arithmetic behind a mortgage is not hard, but it is fiddly, order-dependent and unforgiving of rounding — small modelling errors compound into misleading advice.
Scenario comparison mattered more than any single result: buyers needed to see how deposit, term and rate interact rather than receive one figure with no context.
Calculations had to stay correct as the underlying rules and rate assumptions changed, without every change becoming a code release.

The plan

Our approach.

Modelled the calculation domain explicitly in Django — principal, term, rate, deposit and the derived figures — so the arithmetic lives in one reviewable place rather than being scattered across view logic.
Exposed the calculation engine through Django REST Framework so the same rules serve the interactive UI and any downstream consumer, and so results cannot drift between surfaces.
Moved the heavier scenario work onto Celery, keeping the interactive path responsive while longer comparison runs complete in the background.
Built the front end in HTML and Sass with progressive disclosure: a simple first answer, with the full breakdown of how it was reached one interaction away.
Treated the calculation logic as the part most worth testing, covering the boundary cases — very short terms, very large deposits, rate changes mid-term — where naive implementations quietly go wrong.

The result

The outcome.

Buyers get an affordability picture tied to a specific property rather than a generic estimate.
Scenario comparison is a first-class feature, so the trade-off between deposit, term and monthly cost is visible instead of inferred.
The calculation rules sit in one audited place, which makes changing an assumption a contained edit rather than a hunt through the codebase.
The platform gives the same answer everywhere it is asked, because every surface calls the same engine.

Trusted by our clients

What partners
tell us.

Most clients renew for a second engagement. The ones who don't usually hire someone from our team to run the project in-house.

We are a mental health charity and not tech-minded. We needed to improve our patient registration system but wanted a company that we can relate to. Hassan took the time to meet with me and patiently went through our process to find a personalised solution for us. Hassan's team did a great job streamlining our process and connected our registration system to our patient management software. Whenever we have a glitch, they resolve this quickly. Enigmatix Global provides a personal and efficient service and we have adopted them to be our IT provider.

Tien Kuei

CEO · Power to Live Foundation

I am writing to wholeheartedly recommend Enigmatix Global as a premier bespoke software development agency. Our journey with them has been nothing short of exemplary, far exceeding our initial expectations. While many agencies can develop based on strict specifications, Enigmatix Global demonstrated an incredible commitment to our project that was truly unparalleled.

Robert McPherson

Managing Director · RBM Global Ltd

Dragonfly has been working with the team at Enigmatix Global for several years now on building out our web-based platform and APIs. One of their strengths is quickly building out a multi-disciplinary team to work on the development and delivery of features with different technical requirements. Their support has allowed us to accelerate our development work without building an internal team. They have also worked seamlessly with other third parties both during the acquisition of Dragonfly and development of our mobile app. Enigmatix Global has been instrumental in bringing our ideas to life and providing guidance to help us make the best technical decisions. I would recommend getting in touch with them if you're looking for support.

Rebecca Palser

COO · Dragonfly

Hassan and the Enigmatix Global team have been indispensable partners as we rebuilt our client-facing applications into a single platform. As well as being exceptional technically, they consistently invest the time to understand the product and suggest how it can be enhanced and improved.

David Claridge

CEO · Dragonfly

Enigmatix Global did some great work for Sainsbury's, bridging complex interactions between some older less flexible systems, some rigid vendor solutions and the new systems we were building. The software Enigmatix Global developers wrote stood up well, long beyond the interim scope of it needing to work for 12 to 18 months, and was still in active use 5 years after its creation, since it was part of mission-critical systems.

Fraser Pearce

Senior Engineering Manager · Sainsbury's

Frequently asked

Common questions, answered.

Yes. We provide software development, dedicated engineering teams, offshore development centres and staff augmentation for UK and US businesses. Our head office is in Rochdale, UK. US delivery is remote; we agree working-hour overlap and delivery responsibilities during discovery. Our US service-area pages describe the markets we serve, not US office locations.

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