First Floor Extensions: A Complete Guide

Jan 4

First floor extensions transform homes by building upward – either as a complete two-storey addition or by expanding above existing rooms, most often a garage.

It’s popular to build both floors at once, creating a complete two-storey addition that might house a kitchen below and bedrooms above. It’s one of the most comprehensive forms of extension, capable of adding multiple rooms and substantial space to your property.

If your home already has a single-storey extension or garage, adding just the upper floor can work brilliantly too.

From structural considerations to costs, planning requirements and design choices, there's plenty to think about before starting this extensive project.

Let’s run through everything you need to know about making a first floor extension work for your home.

Ways to Extend Upward

Before planning your extension, look at what you've got to work with.

Most homes offer several ways to build upward, each with different implications for cost, disruption and planning permission.

Building Above a Rear Extension

Many homes already have ground floor extensions – typically housing kitchens or living spaces. Adding a first floor here often makes sense structurally and architecturally.

The foundations might need reinforcement, but you're working with an established footprint that already complements your home. This proves particularly effective when the new upper floor can connect naturally with existing bedrooms.

Converting Above Garages

Integral garages offer prime space for upward expansion, especially when rarely used for cars. It’s popular to extend over the garage while converting the garage itself into a habitable space for your home.

The existing structure provides a ready-made footprint, though walls usually need strengthening to support the living space above.

Access also requires careful thought – the new room should flow naturally with your upstairs layout rather than feeling disconnected.

It’s often possible to convert above garages to add a new story to your home

Utilising Single-Storey Elements

Look for other single-height parts of your home that could support upper floors.

Utility rooms, side returns and even porches sometimes offer opportunities for clever expansion. The key is ensuring new spaces integrate well with existing rooms rather than creating awkward layouts or dead ends.

Building Two Storeys From Scratch

Starting fresh lets you optimise both levels from day one. You can position stairs exactly where needed, align walls for efficient plumbing, and create spaces that work perfectly together.

While potentially more disruptive initially, this approach often proves more cost-effective than extending in stages.

Understanding The Costs of Building a First Floor Extension

Two-storey extensions typically cost between £2,000 and £3,000 per square metre per floor.

For a modest 20-square-metre extension over two floors, expect to pay between £80,000 and £120,000 for the basic build.

Several factors influence these costs:

Ground conditions and foundation requirements
External materials – matching existing brickwork often costs more
Internal specifications like bathrooms or custom joinery
Structural elements such as steel beams and supports
Window and door choices
Roof design and materials

While two-storey extensions cost more upfront than single-storey alternatives, the cost per square metre often works out lower.

You're essentially getting the upper floor at a discount since many major costs – foundations, scaffolding, and roof work – would be needed anyway.

Planning Permission For First Floor Extensions

Planning departments pay particular attention to two-storey extensions.

You'll need a full application that shows how your plans affect nearby houses, from where shadows fall to which windows overlook neighbours' spaces.

The rules vary between councils but tend to focus on scale, privacy and local character.

Planning officers check:

Where upper windows will look out over other properties
How shadows move across gardens and windows as seasons change
Whether your extension's size suits your house and street
If materials and design fit the area's character
Changes to parking, especially when building over garages
Where rain and surface water will drain

If your home is built in a conservation area or other areas where planning guidelines are somewhat more restrictive – which is more common than many assume – you might need to match the new building exactly to your property. This applies to the brick pattern, mortar colour, window style and roof pitch.

Finally, listed buildings need extra permissions with strict controls over what you can build and how.

Design Elements

The best two-storey extensions blend with their original houses while creating bright, welcoming spaces inside.

Your ground floor plan needs to balance supporting walls and beams with open, flowing rooms. Old and new areas should merge naturally, with light reaching deep into existing rooms.

Upstairs layout matters just as much. New bedrooms should connect easily with existing rooms, landings, corridors, etc.

Bathrooms usually work better sitting above ground floor plumbing. Even seemingly minor details like landing space and storage affect how well rooms work together.

As for the exterior, window sizes and positions shape both the view, natural light, and street appearance.

Your choice of brick, stone or render – whether matching existing or purposefully different – affects both planning approval and long-term weathering. A well-designed roof pulls everything together.

Ensure you enable plenty of natural light into your new extension

Building Work

Two-storey extensions typically take 16-20 weeks to build, longer for period properties or unusual designs.

Weather, material delays and unexpected findings behind old walls can all affect timelines.

Great builders plan for these variables while protecting your home throughout construction – particularly if the weather is poor.

The build usually follows this sequence:

Foundation work and ground preparation
Ground floor walls and support structure
Upper floor construction and floor layers
Roof building and weatherproofing
Internal walls and services installation
Final finishes and decoration

The exact process depends on your project, size, complexity, access, and other factors.

Creating Your 1st Floor Extension

A properly planned two-storey extension opens up new ways to use your home while adding substantial value.

At Humphreys and Sons, we've crafted extensions across London, Essex, and surrounding areas, complementing both Victorian terraces and contemporary houses.

We understand how ideas evolve from the first sketch to the final brick. Our team guides you through each stage, balancing creative possibilities with real-world practicality.

Want to learn how your home could grow? Contact us for a free consultation. We'll explore your ideas and what will work best for your property.

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