When you deploy a function app, you create an App Service plan to run it on. When you deploy the next function app, you create another one. It's the default pattern in the portal and in most quickstart templates. It makes sense in isolation.
The problem is that App Service plans have a base cost regardless of how many or how few apps run on them. A Premium P1v3 plan in UK South costs around £120/month whether it's running one function or ten.
What this looks like in practice
In a typical Azure estate that's grown over three years, it's common to find 10-20 App Service plans across production and non-production environments. Some run one function. Some have been empty since the project they were created for ended.
At £120/month per plan, 15 plans is £1,800/month, £21,600/year. Consolidate to three plans (production, staging, dev) and the same workloads run for £360/month. The Azure bill for App Service drops by 80%.
The saving doesn't require changing any application code. It requires moving function apps between plans, which is a configuration change in the portal or a Terraform resource argument.
When isolation genuinely matters
There are legitimate reasons to separate apps across plans. Different scaling profiles that would conflict: one app that needs to scale to 30 instances, another that should stay at two. Apps with different security boundaries or compliance requirements. Apps that consume enough memory that co-tenanting would cause resource contention.
These are real constraints. In most estates, they apply to a minority of apps. The majority of function apps in a corporate environment are small, infrequently triggered, and have no meaningful isolation requirement.
The non-production environment case
NPD environments (dev, test, UAT) are the clearest consolidation opportunity. Multiple environments, each with their own per-project plans, running at full Premium tier cost around the clock including nights and weekends.
Two changes compound here: consolidating plans within the environment and stopping the plans outside business hours. An NPD App Service plan that runs 8am-7pm weekdays spends roughly 40% of the month active. Stopping it the rest of the time cuts that plan's cost by 60%.
Combined with consolidation, the saving on NPD App Service plans is often the quickest £10,000/year win in an Azure estate review.
The Premium tier requirement
App Service Premium (P1v3 and above) is often required to support private endpoints, essential for apps that need to communicate with private backends or be accessed only from within the VNET. This rules out dropping to Basic or Standard tiers even if utilisation is low.
That's not an argument against consolidation. Multiple functions on a single Premium plan still costs one Premium plan. The tier is the floor; the number of plans on top of that is the variable.
App Service plan consolidation is consistently one of the higher-value, lower-effort findings in our Azure cost reviews. Our free cost assessment includes a full review of your App Service estate.