What Are the Building Regs for Windows & Doors in 2026?

17th December 2025

In 2026, building regulations for windows and doors in the UK focus on energy performance, ventilation, safety, and security. Building Control will check that your chosen system, the finished installation, and your supporting evidence all match the relevant Approved Documents [1].

Issues often show up during inspection. For example, windows might meet the thermal target on paper, but the ventilation route has changed. Doors might fit the brief, but the security evidence is missing. These problems usually come from late decisions made after manufacturing or installation has started. In practice, the 2026 building regs require you to choose systems that meet set performance standards and keep the evidence needed for sign-off.

This guide is for architects, builders, and developers. It covers where projects often slow down and what you should confirm early when choosing aluminium windows and doors.

Part L & Part F: What You Need to Specify to Stay Compliant

Approved Document L sets the minimum energy performance standards for windows, doors, and rooflights [2]. Building Control uses it to check if heat loss through openings is acceptable for your building type and project stage.

Part L looks at U-values for the whole window or door, so both the frame and glazing performance count together. This is important for aluminium systems, since the frame design has a big impact on the final thermal result.

Approved Document F works with Part L and covers ventilation [3]. Replacement windows are especially affected by Part F because modern units often make buildings more airtight. If ventilation is reduced, the work will not meet the rules.

The Four Key Decisions to Make Before Manufacturing

• The target whole window U-value for the elevation and building type
• The agreed glazing specification, including any solar control requirements
• The background ventilation route, including whether trickle ventilators are required
• Installation detailing that supports airtightness and continuity at the reveals

Approved Document F also states that night-latch positions do not count as background ventilation, and you must keep the same level of ventilation when replacing windows. If façades face ongoing noise, the guidance allows noise-reducing ventilators when needed.

These performance choices are closely connected. You need to agree on frame design, glazing, and ventilation together to make sure the planned performance is achieved on site.

If you are choosing a flush casement aluminium profile, MAXi2 Flush Casement Windows are a good example for matching appearance with performance early in your project.

What Happens if This Is Not Resolved Early

If you change your Part L or Part F assumptions late in the project, the effects are rarely limited to one area:

Changing glazing can affect U-values, ventilation changes can impact frame design, and installation details might no longer match the original assessment.

This is often where projects get delayed. Making changes late increases the risk of extra work and makes it harder to provide the evidence Building Control needs.

Approved Document Q: What Counts as Secure by Design

Approved Document Q applies to new dwellings, including dwellings created through a material change of use [5]. It sets out what Building Control will normally accept as a reasonable provision to resist unauthorised access through easily accessible doors and windows.

For project teams, it is easier to meet security requirements when you make them part of the specification, not something to fix on site. Document Q lists accepted options based on tested systems, usually doorsets and windows that have passed PAS 24 tests.

Once the layout and access strategy are agreed, it helps to confirm the following before procurement:

• Which openings fall within the Document Q scope, based on height and ease of access
• The tested compliance route for each doorset and windowset, including the intended hardware set
• What evidence will be retained for Building Control to review at sign-off

The guidance also sets out clear rules for door hardware. For example, letter plates should be designed and placed to lower the risk of key fishing, with their size and position matching the standards listed in Document Q.

When entrance doors are a main feature of the façade, MAXi Designer Doors help you coordinate door style, hardware, and installation needs with your chosen security approach from the start. For projects with large, glazed openings, MAXislide Sliding Doors let teams confirm the security plan before finalising thresholds and fixing details.

If you need flexible access, aluminium bifold doors are a common choice. Agreeing early on thresholds, hardware, and compliance evidence helps avoid late redesigns after openings are made.

Working with Alimax on A Compliant Specification

To meet the 2026 building regs for windows and doors, make early decisions and stick to them through manufacturing and installation. Thermal performance (Part L), ventilation (Part F), and security evidence (Approved Document Q) must all match the approved design and be backed up by clear documents for Building Control.

Alimax partners with architects, builders, and developers to help set performance targets, choose systems, and prepare evidence packs. Getting involved early helps confirm the compliance route before you order materials and reduces late changes after openings, thresholds, and fixing details are set.

Call 01425 205465 or book a consultation to find the best aluminium windows and doors for your project.