How To Maximise Daylight In A London House Extension

In this blog post we explore how to maximise daylight in house extensions, particularly in dense urban settings like East London. Featuring our newly completed project Reflective House, we walk through smart design strategies, planning tips, and how to meet Building Regulations Part L, even with large areas of glazing.

The Challenge

The original layout of Reflective House was typical of many London terraces: the front was south-facing and bright, but the rear had limited access to sunlight. Our goal was to transform a dark rear living space into a bright, open-plan kitchen and dining area, without losing connection to the rest of the home.

By preserving the south-facing front window and maintaining a dual-aspect layout, natural light from both ends of the house was allowed to flow through the home. This approach improved light levels and created a more balanced, inviting interior.

Smart Glazing Design

Adding more glass isn’t always straightforward due to recent changes in energy regulations. At Reflective House, we were able to install a large, carefully positioned rooflight spanning the entire kitchen zone, to ensure even daylight coverage by utilising the area-weighted U-Value calculations in Part L of the Building Regulations. This is explained in further detail below.

We also replaced the small rear door with a full-height glazed opening, visually connecting the interior to the garden and increasing perceived space. A generous upstand around the rooflight and a parapet roof allowed us to increase ceiling height while staying within local planning constraints.

Connecting Kitchen and Garden

A defining feature of Reflective House is the trio of sliding aluminium doors, installed across both the kitchen extension and the garden studio at the rear of the plot. These sliding doors open fully, allowing:

• Uninterrupted views from the front window to the back of the garden studio

  
  

• Seamless physical and visual connection between the main house, the garden, and the garden studio

  
  

• Enhanced daylight penetration, drawing reflected garden light back into the living space

  
  

When open, the doors make the garden, studio, and extension feel like a continuous environment. This transparency was instrumental in ensuring the rear, north-facing spaces felt vibrant, open, and bright. From the front hallway, you can now see all the way through to the studio at the end of the garden.

At the rear, we installed three-part sliding aluminium doors—one set in the extension and another in the new garden studio. When opened, they blur the boundary between inside and out. From the front hallway, you can now see all the way through to the studio at the end of the garden. It’s a game-changer for how the space feels: bright, spacious, and totally connected.

Interior Glazing

Our clients needed a quiet workspace which remained connected to the house. So we created a glazed acoustic screen between the kitchen/living space to create a home office. This preserved a sense of openness and daylight flow whilst offering sound separation. It was also critical in maintaining the dual-aspect daylight strategy, allowing light to penetrate from front to back.

Volume and Structure

To maximise the feeling of space, the extension was designed to be taller than the existing ground floor. We exposed the timber roof rafters, creating rhythm, texture, and a warm natural feel where light could be filtered through.

Letting The Sky In

Instead of a projecting roof or canopy, we chose a flat parapet roof for both the kitchen extension and garden studio. This avoids unnecessary shading and allows unobstructed access to the northern sky, drawing in diffused daylight throughout the day - crucial for a rear extension which does not have access to direct sun.

Building Regulations Part L Explained

Many homeowners worry about exceeding the 25% glazing limit under Building Regulations Part L. At Reflective House, we tackled this using an area-weighted U-value calculation, allowing more glazing while still meeting statutory energy efficiency standards.

This method compares the overall thermal performance of the proposed extension against a notional extension of the same size and shape, but built to the minimum U-values required by Approved Document L1B.

If the proposed design has an equal or better (i.e., lower) area-weighted U-value than the notional compliant version, then it meets Part L, even if the glazed areas exceed 25%.

 Our strategy included:

• High-performance glazing with low U-values

• Upgraded insulation to floors, roof, and walls to above the minimum requirements – the more that these elements can be improved – the more glazing that can be incorporated.

• Thermal bridge minimisation at all junctions

This holistic approach allowed us to meet Part L while achieving a bright, open design.

Whole House SAP Calculations

While an area-weighted U-value calculation is ideal for many extensions, there’s a more flexible - but more involved - option available: the SAP whole-house calculation.

SAP (Standard Assessment Procedure) is the UK government’s approved method for calculating the energy performance and CO₂ emissions of a dwelling. The full SAP route evaluates:

• The entire house, not just the extension

• The performance of the proposed design against a notional extension built to minimum standards

• The carbon emissions of the whole home to verify compliance

This route typically requires a qualified energy assessor using SAP 2012-approved software.

This approach is particularly useful when:

• You’re undertaking larger or more complex alterations with energy upgrades throughout

• You want greater freedom in glazing ratios and fabric performance

• You're combining a house renovation with an extension

  
  

Planning a House Extension in London?

At Unagru, we specialise in transforming London homes through intelligent design. Whether you're planning a side return, loft, or rear extension, we can help you:

• Maximise daylight

• Meet planning and Building Regulations

• Create calm, contemporary living spaces

Get in touch today for a complimentary consultation to start your project.