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Double-storey extension to a traditional London house

Double-Storey Extensions: How to Add Space Without Overpowering the Original House

Discover how a carefully designed two-storey extension can add valuable rooms upstairs and downstairs while remaining visually subservient to the original property and sympathetic to its character.

A double-storey extension can be one of the most effective ways to add substantial value and usable space to a home.

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Unlike a single-storey extension, it does not simply enlarge the kitchen or ground-floor living area. It can also provide one or two additional bedrooms, a larger bathroom, an en-suite, a home office or a more practical family layout upstairs.

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For the right property, a well-designed two-storey extension can transform a modest house into a far more comfortable long-term home.

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However, double-storey extensions need to be designed carefully.

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The most successful examples do not look like oversized additions bolted onto the side of the house. They appear balanced, proportionate and sympathetic to the original building.

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This is particularly important where planning permission is required.

Which Properties Suit Double-Storey Extensions?

 

Double-storey side extensions are most commonly seen on:

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  • Semi-detached houses

  • End-of-terrace houses

  • Detached houses

  • Corner plots

  • Wider suburban properties

  • Homes with generous side access

  • Properties with an existing garage or side projection

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Semi-detached and end-of-terrace houses are particularly suitable because they often have enough land at the side to accommodate a substantial extension.

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The additional width can allow the ground floor and first floor to be improved at the same time.

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A successful extension may create:

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  • A front study or home office

  • A playroom

  • A utility room

  • A larger kitchen

  • A downstairs WC

  • A boot room

  • One or two additional bedrooms

  • An en-suite bathroom

  • A larger family bathroom

  • Better storage

  • Improved circulation upstairs

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The precise layout depends on the shape of the property, the available width and the position of the staircase.

Why Planning Design Matters

 

A double-storey side extension can have a much greater visual impact than a single-storey addition.

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This is why planning departments usually consider the proportions and roof design carefully.

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One of the main planning concerns is the risk of creating a terracing effect.

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This can happen when a side extension makes a semi-detached or end-of-terrace house look excessively wide or brings the first-floor wall too close to the neighbouring property.

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The extension may appear to close the visual gap between houses and change the rhythm of the street.

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A carefully designed extension should avoid overpowering the original building.

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The aim is usually to make the extension appear slightly subservient to the main house.

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Setting the First Floor Back

 

One of the most effective ways to make a double-storey side extension feel more sympathetic is to set the first-floor front wall back from the main elevation.

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A setback of around 500 mm is often a sensible starting point, although the right dimension will depend on the property and local planning context.

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The ground floor may still extend forward to align more closely with the original house.

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The first-floor setback helps distinguish the new addition from the original building.

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It also reduces the visual bulk of the extension when viewed from the street.

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This can make a significant difference during the planning process.

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Lowering the Roofline

 

The roof should also be designed to remain slightly lower than the main roof.

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A roof ridge set down by around 500 mm can help the extension read as a secondary element rather than an oversized continuation of the main house.

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Where possible, the roof pitch should match or closely reflect the original roof.

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This creates a more cohesive appearance.

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The roof design may include:

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  • A matching pitched roof

  • A hipped roof

  • A gable roof

  • A lower ridge line

  • Matching tiles or slates

  • Similar eaves and fascia details

  • Consistent guttering

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The exact approach will depend on the style of the property.

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A 1930s semi-detached house may benefit from a hipped roof that follows the proportions of the existing building.

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A more traditional end-of-terrace property may suit a gable or pitched arrangement.

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Matching the Original House

 

The best double-storey extensions are often the ones that do not immediately look like extensions.

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The detailing should feel natural.

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This usually means carefully matching:

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  • Brickwork

  • Render

  • Roof tiles or slates

  • Window proportions

  • Window positions

  • Window frames

  • Sills and lintels

  • Fascia boards

  • Guttering

  • Roof pitch

  • Eaves levels

  • External detailing

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The new windows should be aligned properly.

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Where possible, they should reflect the proportions and spacing of the original windows.

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Poorly positioned windows can make an otherwise good extension feel awkward.

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Likewise, mismatched brickwork or an incorrectly proportioned roof can make the addition look obvious and disconnected from the main house.

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A successful extension should feel as though it has always been part of the property.

Ground-Floor Layout Ideas

 

The ground floor of a double-storey side extension can be used in several ways.

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The best arrangement depends on whether the property already has a rear extension and how the family uses the house.

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A common layout is to create a separate room at the front.

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This may be used as:

  • A home office

  • A study

  • A playroom

  • A snug

  • A guest room

  • A hobby room

  • A small sitting room

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This can be particularly valuable in a busy family home.

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A separate office allows someone to work from home without taking over the dining table or front reception room.

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A playroom can also help keep toys and clutter away from the main living areas.

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Using the Rear of the Extension

 

The rear section of the ground floor may be used to improve the kitchen or utility areas.

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Popular options include:

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  • A larger kitchen

  • A utility room

  • A pantry

  • A boot room

  • A downstairs WC

  • A shower room

  • Additional storage

  • A direct route into the garden

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In some homes, the side extension can connect with a rear extension to create a much larger kitchen-diner.

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In others, the side extension may provide the practical supporting rooms that allow the main kitchen to feel cleaner and more spacious.

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A utility room can be particularly useful.

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It allows washing machines, dryers, cleaning products and household storage to be removed from the main kitchen.

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Upstairs Layout Ideas

 

The first floor can create some of the most valuable improvements.

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Where the staircase layout allows, a doorway from the existing landing may lead directly into the new extension.

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A small internal lobby can then provide access to rooms at the front and rear.

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This may allow the extension to contain:

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  • Two additional bedrooms

  • A bedroom and en-suite

  • A larger principal bedroom

  • A bedroom and study

  • A family bathroom

  • A dressing room

  • Additional storage

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A front and rear bedroom arrangement often works well.

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Each room benefits from an external window, and the central lobby provides practical circulation.

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The front bedroom may suit a child, guest or home office.

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The rear bedroom may benefit from quieter views over the garden.

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Creating a Larger Principal Bedroom

 

Some homeowners prefer to use the first-floor extension to create one larger bedroom suite.

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This might include:

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  • A generous bedroom

  • An en-suite shower room

  • Built-in wardrobes

  • A dressing area

  • Additional storage

  • Garden views

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This can be a good option where the house already has enough bedrooms but lacks a comfortable principal bedroom.

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The layout needs to be designed carefully to make the best use of the width and available windows.

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Extending an Existing Garage

 

Many suburban homes already have an attached garage.

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This can provide a useful starting point for a double-storey extension.

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The garage may be retained, converted or replaced as part of the project.

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Possible options include:

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  • Converting the garage into a study or playroom

  • Adding a first-floor extension above the garage

  • Rebuilding the garage with improved foundations

  • Creating a front room with a new window

  • Adding a utility room behind the former garage

  • Retaining storage space at the front

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However, it is important to confirm whether the existing foundations are suitable.

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An older garage may not have been constructed to support an additional storey.

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Structural assessment and Building Regulations design will usually be required.

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Roof Design Options

 

The roof is one of the most important visual elements.

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The extension should normally reflect the main house rather than compete with it.

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Hipped Roofs

 

A hipped roof often suits 1930s semi-detached houses.

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It can help the extension feel balanced and reduce its visual bulk.

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The new roof can follow the general form of the existing house while remaining slightly lower.

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Pitched Roofs

 

A conventional pitched roof may suit many suburban homes and end-of-terrace properties.

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Matching roof tiles and ridge details can help the extension blend in.

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Gable Roofs

 

A gable roof can work well where it reflects the original house or where the extension needs a stronger architectural feature.

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However, the proportions need careful thought.

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A large gable can appear dominant if it is not designed properly.

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Roofs Above Single-Storey Sections

 

Some double-storey extensions also include a smaller single-storey section.

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This may have:

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  • A flat roof

  • A lean-to roof

  • A shallow pitched roof

  • A small porch roof

  • A rooflight

  • A lantern

  • A canopy

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This can help create a more varied and interesting elevation.

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Front Elevation Design

 

The front elevation is particularly important because it determines how the extension will appear from the street.

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A good front elevation should feel balanced.

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The extension should complement the house without making it look too wide or top-heavy.

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Things to consider include:

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  • Whether the first floor should be set back

  • The width of the extension

  • The position of the windows

  • The relationship between the new and existing rooflines

  • The design of any porch or canopy

  • Matching materials

  • The position of the original front door

  • The treatment of the driveway and landscaping

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In many cases, the original front door should remain in its existing position.

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The new extension may simply have a well-proportioned window on the ground floor and another aligned window above.

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This helps keep the extension simple and visually balanced.

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Avoiding a Bolt-On Appearance

 

A common mistake is to design the extension as a completely separate block.

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This can make the house look disjointed.

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The addition may feel like an afterthought if:

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  • The roofline is too high

  • The brickwork is poorly matched

  • The windows do not align

  • The first floor projects too far forward

  • The gutters and fascia boards do not line up

  • The extension is too wide

  • The porch feels unrelated to the house

  • The new room arrangement creates awkward elevations

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The best designs are carefully proportioned from the beginning.

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They work both internally and externally.

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Planning Permission

 

A double-storey side extension will normally require planning permission.

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The local authority will consider factors such as:

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  • The width of the extension

  • The first-floor setback

  • The roof height

  • The roof design

  • The relationship with neighbouring properties

  • The gap between houses

  • The risk of a terracing effect

  • Overlooking

  • Loss of light

  • The character of the street

  • The materials

  • Any local planning policies

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A setback and lower roofline can strengthen the design, but every property needs to be considered individually.

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Local planning policies vary, and there is no automatic guarantee of approval.

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A carefully prepared design is therefore essential.

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Building Regulations and Structural Design

 

Building Regulations approval will normally be required.

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Structural calculations may also be needed.

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The structural engineer may need to consider:

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  • Foundations

  • Load-bearing walls

  • Steel beams

  • Roof design

  • Lintels

  • Floor joists

  • Existing garage foundations

  • Drainage

  • Insulation

  • Fire safety

  • Ventilation

  • Stair and landing arrangements

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Where the extension changes the internal layout significantly, the Building Regulations drawings should be coordinated carefully with the structural design.

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Party Wall Considerations

 

Party wall procedures may apply if the extension involves excavation close to neighbouring foundations or work near a shared boundary.

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This is particularly relevant where the extension is built close to the side boundary.

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The position should be reviewed before work starts.

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Thinking About the Driveway and Front Garden

 

A side extension can also affect the front garden and driveway.

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The extension may reduce parking space or change the route to the front door.

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This should be considered during the design stage.

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Questions to think about include:

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  • Will the driveway still accommodate a car?

  • Is the front door easy to find?

  • Does the entrance feel welcoming?

  • Is there enough space for bins?

  • Will drainage be affected?

  • Does the landscaping soften the new extension?

  • Should the porch be altered or retained?

  • Is side access still required?

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A good design considers the whole property rather than focusing only on the additional rooms.

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Getting Quotations from Builders

 

Builders can provide more reliable quotations when the project is clearly defined.

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A good design package should show:

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  • Existing and proposed floor plans

  • Existing and proposed elevations

  • Roof plans

  • Sections

  • Window positions

  • Materials

  • Structural openings

  • Internal layouts

  • Any utility or bathroom arrangements

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For more accurate quotations, Building Regulations drawings and structural calculations may also be helpful.

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This makes it easier to compare quotations on a like-for-like basis.

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Finding the Right Professionals

 

A double-storey extension may involve several different professionals and trades, including:

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  • Architectural designers

  • Planning consultants

  • Building Regulations designers

  • Structural engineers

  • Building Control professionals or approved inspectors

  • Party wall surveyors

  • Builders

  • Roofing contractors

  • Window suppliers

  • Bricklayers

  • Electricians

  • Plumbers

  • Kitchen designers

  • Bathroom installers

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The Architectural Partner Network (APN) helps homeowners access vetted and approved professionals for different stages of their projects.

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Whether you are exploring ideas, preparing for planning, comparing quotations or getting ready to build, the right advice can help you make better decisions and avoid costly mistakes.

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Tell Us About Your Double-Storey Extension

 

A double-storey extension can add valuable rooms upstairs and downstairs while making a home much more practical for long-term family life.

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You may already know that you want a front study, larger kitchen and two additional bedrooms, or you may still be working out what is possible.

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Tell us a little about your property, your ideas and the stage you have reached. We can help point you towards the most appropriate professionals for the next step.

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