Modular web design vs custom development: which builds higher-converting websites?

Both labels cover a wide range of work. A modular site can be assembled from a carefully built library or a poor one, and a custom site can be coded well or badly. Two sites with the same label can differ in their copy, their offer, their speed and the amount of work done on them after launch.

So knowing which label applies tells you little about how the finished pages will perform for your buyers. A comparison that would settle it has to hold the copy, the design and the offer steady, and a label can't do that for you.

Lean Labs' account of modular web design describes assembling pages from complete, reusable sections and then customizing each one with its own copy and images. A hero section in that approach arrives with a heading, a call to action and image cards already arranged. The agency builds this way on HubSpot with a product called Sprocket Rocket.

Custom development is often pictured as the opposite, with every page designed and coded as its own piece. It doesn't have to work that way. HubSpot's module documentation calls modules reusable components that go into templates or onto pages, and it says developers can create their own "for everything from testimonials to photo galleries." Each use of a module can carry its own field values, so one testimonial module can show a different customer wherever it appears. A HubSpot theme then packages a developer's templates, modules and style settings into the system editors work in.

A site built that way is custom-coded and modular at once. Design Systems 101 from Nielsen Norman Group (NN/g) describes reuse from the design side. It defines a design system as a set of standards with reusable components and patterns, and it says a team can adopt an existing system, adapt one or create its own. A system a team creates for itself is custom work, and its components are built to be reused.

The choice that affects your site is between reusable parts and page-specific builds. Either one can come from a purchased library or from your own developers.

Suppose a B2B software company has 30 product and industry pages, and each one carries a customer testimonial. The sales team reports that buyers keep asking whether anyone in their own industry uses the product. The marketing team decides every testimonial should name the customer's industry and link to the full story.

Now suppose two versions of that site. On one, the testimonial is a single shared module used on all 30 pages, and a change to the module's fields reaches every page that uses it. On the other, each page has its own independently built layout.

On the first site, a developer adds the two fields to the module and an editor fills them in page by page. On the second, someone makes the same structural change in each of the 30 layouts. Both teams end up with the same pages, and the first team repeated less work. That saving doesn't decide when the change ships or how many changes a team makes, because those depend on the team's time and priorities.

NN/g credits a design system with consistency and less repeated work, and it also says the system takes time to maintain and to teach. A shared part carries a risk in the other direction too. HubSpot's content staging guide warns that changes to shared templates and global content affect the live pages that use them, so a mistake in one of them reaches the live pages that use it. And when a page needs a layout that no existing section or setting can produce, that layout is new development on either kind of site.

Conversion gains can follow specific changes to a page, such as what it says, offers or asks of the visitor, and you look for candidates in your own evidence. Lean Labs' growth-driven design service describes drawing on page behavior, form results, CRM lifecycle data and sales feedback to decide what to change. It ranks the candidates by expected pipeline impact, by buyer relevance and confidence, and by effort.

Effort is where the build method enters that ranking. When a change repeats the same structural work across many pages, as in the example, a shared section means doing that work once. That's a reason to prefer reuse on a site that expects frequent changes, and it guarantees nothing about results.

A test can show whether one change did better than the version it replaced, and some tests end without a clear answer. HubSpot's page A/B test, available on Content Hub Professional and Enterprise, shows two versions of a page at one URL and sends half the visitors to each. In the example above, it could compare the old testimonial section with the new one on a single product page. It doesn't compare modular with custom, because both versions were built the same way. We cover the wider question in whether a website redesign will fix low conversion rates, and the testing work in turning user insights into hypotheses and experiments.

Reusable sections fit a site with many pages of a similar kind and a team that expects to keep changing them. When pages share a structure, a section built once gets used again on each of them. Launch timing is its own question, which we cover in how long it takes to launch a growth-driven design site.

Page-specific work fits a small site that will rarely change, or a single page whose layout does a job no shared section could do. The two can sit on one site, with a library for the standard pages and a few pages built by hand.

When you're comparing agencies or approaches, we'd look past the label. Find out who can change a section that appears on 30 pages once the site is live, and what that change takes.