Building an Extension: A Detailed Guide

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Written by Taylan Tahir

August 2021

Written by Taylan Tahir

August 2021

An extension to your home is a significant investment into your lifestyle and happiness.

Whether you live in a detached, semi-detached or terraced house, a successful extension will do much more than add floor area and value to your home. It should reflect your personality and, if done well, will lift your everyday.

This article addresses some of the opportunities and challenges you might face when planning and constructing an extension for your home.

Black Box – A first floor rear extension in the Barnsbury Conservation Area, Islington.

If you are new to extension and renovation projects it can be difficult to know where to start. This checklist is an overview of the main considerations involved with links to further detailed information:

1. Commission a Measured Building Survey

The basis for every construction project is a set of scale drawings. It is essential that information used is accurate from the outset to avoid any issues down the line. These drawings will be used for planning and construction purposes. An architect will require these drawings to begin work. Lead times for measured building surveys can range from 3 to 6 weeks so it is worth organising this early.

2. Prepare a Project Brief

A project brief forms the basis for initial conversations with a designer and is a fundamental first step for any extension or renovation project. It sets out key requirements and aspirations for the project and helps to ensure the end result meets your goals. Find out more.

3. Calculate the Project Budget

This will need to include VAT on the construction costs as well as consultants, statutory fees and a contingency. To help plan your own project budget, read more about the costs involved in constructing a house extension here [LINK].

4. Assess Project Budget Against Goals

Ensure you find out early on if your budget matches your aspirations for your project.

5. Estimate Project Timeline

Construction projects always take longer than you first anticipate. If you have any important life events coming up, include these in your brief and ensure you plan around them. Read more about the timescales involved in domestic residential project here.

6. Build the Project Team

A great (and experienced) project team will produce a great project. The team can include Architect, Structural Engineer, Quantity Surveyor, Planning Consultant, Planning Officer, Party Wall Surveyor, Approved Inspector, Main Contractor, Sub Contractors, Suppliers. The role of the Architect is one of lead designer, it is their responsibility to coordinate the project team for the duration of the project and ensure everyone is working from the same page. Find out if you need an architect for your project.

7. Design

An investment in a great design will improve your lifestyle and happiness for years to come as well as adding significant value to your property. You wouldn’t buy a car or a house without seeing it, and the same goes for an extension. Ensure you work with a designer that can visualise your home in 3D. The better you understand what you’ll end up with the more you’ll be able to engage with the process and make better decisions along the way.

8. Check Statutory Planning Requirements

Establish whether your extension will require planning permission or if it can be constructed under Permitted Development. Learn more here.

9. Party Wall Agreement

Ascertain if your proposed extension would require a Party Wall Agreement. Ensure Party Wall proceedings begin ahead of construction to avoid delays.

10. Check Insurance Requirements

Ensure your home/building insurer is kept updated with any alterations being made to your house. Joint Names Insurance between owner and contractor is a requirement for many building contracts involving an extension or renovation to a domestic property. 

11. Find the right Contractor

Finding a contractor that is exactly the right fit for your project can be a challenging part of the process. As well as agreeing on a price, ensure you and your Architect perform thorough due diligence. Ask for information on the past 2-3 years turnover and speak to architect/client references.

12. Use a Building Contract

 It is essential you sign a building contract with your contractor for peace of mind and to reduce risk during the construction phase. JCT contracts are robust and one of the most commonly used for homeowners.

13. Confirm the Retention & Defects Liability Period

There are two mechanisms in a building contract that will give you peace of mind that the contractor will complete the works to a good standard and also return to fix any defects.

The ‘retention’ is a sum of money (usually 5% of the contract value) that is held back during the duration of the works. Following the completion of the project, half of this sum (2.5%) is paid to the contractor.

Following this, the ‘defects liability period’ begins and lasts 6 months (or longer on larger projects). At the end of this period the client reports any defects that have arisen and the contractor returns to rectify them. When all defects are rectified the final half of the retention is paid to the contractor

Planning approval for a full width rear extension at Zig-Zag House in Queen Park, London

Building an Extension: 15 House Extension Ideas

1. A single storey side return or rear extension

Even the smallest extension brings a big opportunity to add better space and light. This type of extension is sometimes referred to as a kitchen extension (if they accommodate a new kitchen in the additional space).

2. A wraparound extension

Utilising the full width and depth of the existing house footprint maximises additional floor area. This is one of the most effective design options for creating outstanding open plan living spaces.

3. A fully glazed extension

A conservatory, orangery or garden room are more traditional forms of glazed extensions. Alternatively, a more contemporary, minimal ‘glass box’ extension could provide the ultimate connection to the outdoors. Always consider aspect, solar shading and natural ventilation as fully glazed spaces are likely to overheat in the summer months.

Planning approval for a full width rear and partial first floor extension at Barnsbury House, Islington.

4. Internal courtyards

Can be introduced as combination of side and rear extensions to bring additional light into the existing ground floor as well as bringing the garden into the living spaces.

5. Mirror your extension with your neighbour

This strategy is applicable if you live in a semi-detached or terraced house. Combining the design of your extension with your neighbours is a great option if you are both considering an extension at the same time. You’ll likely be able to achieve permission for a larger footprint/roof height as there will be no impact on the neighbouring property. Another bonus is you’ll be able to share some of the costs along the way.

6. Front extension

This is less common form of extension and can be a non-starter in Conservation Areas due to the impact on the street. Extending beyond the front building line of your house provides an opportunity to enlarge and amalgamate a garage into the house or add a porch.

7. L-Shaped or Stepped Extension

Extending further into the garden on one side of your house provides an opportunity to create a sun trap for an east or west facing house.

8. Multi/Two Storey Extension

Adding habitable space at first floor traditionally offers an additional bedroom, bathroom or study. Alternatively new vertical connections can be created between ground, first or second floor through the use of mezzanine and double height spaces.

Planning approval for a two storey side and rear extension to a detached house in Sprucedale Gardens, Croydon

9. Loft Conversion

Not every roof can accommodate a loft conversion. Some are too shallow and fundamentally do not have the head height. It is worth checking Google Maps to see if anyone on your road has converted their loft already to give you an idea if it could be possible.

10. Roof Extension

If you need extra head height or floor area in order to create additional accommodation in your roof space you could consider a roof extension. This could be as straightforward as adding a dormer window to an existing roof or a more complex rebuild of the whole roof to raise the height of the existing ridge/eaves or a mansard extension. Roof extensions come in different size and forms – An L-Shaped dormer or a ‘Hip to Gable’ designs can help you to maximise the capacity of your roof extension.

11. Basement Conversion

Existing basements or cellars can be converted to be used as habitable space. There can often be limitations with head height and a lack of windows as well as issues with damp to overcome.

12. Basement Extension

Excavating is a good (albeit a very expensive) method to gain additional floor area. You’ll generally find basement extensions in locations where properties are more valuable and there is an economic argument to do so. Basement extensions always have more onerous demolition/structural requirements (e.g. breaking away existing foundations and underpinning) and can also be a more risky proposition at planning, requiring basement impact assessments. An alternative to a full basement extension would be to excavate areas of new extension to achieve greater ceiling heights than the existing house.

13. Garage Conversion

If your house benefits from a garage it may be possible to convert the existing structure into habitable space and, in some cases, add a connection to the house. These spaces are often best utilised as an office/study or a gym/exercise studio.

14. Combination

A mixture of extensions to different parts of your house (e.g. roof, side at first floor above a garage and a single storey rear extension) could give you the ultimate flexibility. One of the biggest challenges here is to create a design that works harmoniously with the existing house.

Garden Studio in Queens Park, London

15. Outbuildings/garden studios

If you benefit from a garden or some land around your home you may be able to construct a new outbuilding. Garden studios are an increasingly popular way to add ancillary floor space to a house and in many cases fall under permitted development.

Building an Extension: Frequently Asked Questions

How much will it cost to build an extension?

In our experience the cost of extending and refurbishing a house in Greater London costs between £2,500 – £3,000 p/sqm. A double storey extension would be a lower cost per sq.m due to economy of scale with a larger construction project (shared foundations, services etc).

Construction costs for house extensions can vary widely depending on a range of factors including:

  • Spec of fixtures and finishes (Particularly the choice of glazing, kitchens, bathrooms, underfloor heating)
  • Site constraints (access to site for construction, proximity to neighbours, space for site welfare etc)
  • Below ground conditions (soil type, location of drainage and utilities)
  • Type of construction system used (timber frame, concrete frame, steel frame etc)
  • Type and size of extension (Single/2 storey, basement, loft etc)
  • External landscaping requirements
  • Location (Construction in Greater London is generally higher than other areas of the UK)

These are very high-level cost estimates and are only appropriate for very early-stage budgeting.

How to build an extension cheaply (or, how to save money when building an extension)?

If you are hoping to reduce the overall cost of your extension and make some savings along the way there are a few key areas you should consider:

  • Build smaller but better spaces and do more with less. It is not always essential to maximise additional floor area. Intelligent design on a smaller footprint can still meet your goals whilst making the most out of your budget.
  • Reduce the specification of fixtures, finishes and fittings by adapting off the shelf or proprietary products instead of designing bespoke.
  • If you are demolishing part of the existing building re-use and recycle as much as you can. Not only is does this reduce the embodied carbon of your new extension but saves you money on new materials.

Click here to read more tips on how to save money when building an extension.

Do I need an architect for my extension?

The short answer to that question is – No. You don’t.

 There is no legal requirement in the UK to employ an Architect for construction projects. The vast majority of new housing is procured without any active input from an architect. Unfortunately, the majority of new housing is also sub-standard and criticised as monotonously uniform.

 The overall cost of a well-designed and well-built home is similar to the cost of a poor one. Equally, the cost of employing an architect is negligible over the lifetime of a building, but the added value is evident at the start and continues to accrue.

 An Architect will:

 Interpret your brief and identify opportunities.

  • Provide professional advice in relation to statutory requirements and additional consultants.
  • Bring planning experience and reduce risk.
  • Ensure compliance with regulations.
  • Look out for your money during the design and construction stages.
  • Reduce risk and provide peace of mind.

 An extension is a significant investment in your health and happiness. We’d argue that in almost every project the experience and skillset an architect brings will add value. It’s up to you to decide whether the investment in a professional is justified.

 If you are interested to learn more about the alternatives to using an architect read our article [LINK]

Will building an extension add value to my home?

For most people their house is their primary financial asset. The idea of adding a beautiful extension to your house that will improve your quality of life as well as increasing its value is an alluring one. Any type of extension will always add some value to your home; however, the challenge is deciding on the right investment for your lifestyle and your property. A house extension in an outer London suburb is going add a different level of value than the equivalent to a property in central London.

 In order to assess the value an extension could add to your home, consider all of the finances involved. Ask the question, “will doing works to my house increase its value by more than the amount I will spend?”. To answer this, you’ll need to research and understand two pieces of information – the potential value of your house after extending vs. the total costs involved in doing the works. [LINK – cost of an extension]

 Look for properties on your road and nearby that have similar extensions and note their recent sale prices. Ask local estate agents about potential values for extended properties in your area. If you are aiming to construct something with high architectural quality, and different to the norm, we would recommend talking to more specialist estate agencies such as the Modern House  or Aucoot. These agencies specialise in selling design focused properties. Between 2014-2017 well-designed homes advertised by the Modern House sold between 10-19% more than a ‘regular’ comparable. Read full article

An extension is not only an opportunity to add better space but also to make wider efficiency improvements to the existing building. Improvements to thermal performance and energy efficiency will reduce the energy (and ongoing cost) required to run your home. This is valuable in itself.

You might find this article and flow chart helpful to decide whether it would be worth investing in your home or moving. [LINK].

What size extension can I build without planning?

In order to answer this question, you’ll need to start by assessing whether Permitted Development (PD) rights would apply to your proposed extension.

In order for your proposed works to fall under Permitted Development they’ll need to meet some specific criteria that affect floor area, scale, location and appearance. These planning rules vary for different types of properties:

  • Semi-detached house extensions [LINK]
  • Terraced House extensions [LINK]
  • Extensions to flats or apartments

 In any event, it’s always a good idea to check with your local authority before commencing construction as PD rules vary in certain areas where rights have been withdrawn. For example, homes in conservation areas, listed buildings or a property that has already benefitted Permitted Development are all ineligible.

 Some type of permitted developments also require notifying the local planning authority via a ‘Prior Approval’ before commencing construction. If you were to pursue the route of a Permitted Development extension, we would recommend that a Certificate of Lawfulness of Proposed Use or Development (CLOPUD) is sought from the local authority in order to create a formal record of the works and their legitimacy.

 To understand how PD rights would affect your extension read our article.

House extension planning rules

In many cases the design of an extension that is distinct from the existing house and is contrasting in appearance (form and materials) would fall outside of Permitted Development rights regardless of scale. In this instance planning permission would be required.

While there are no hard and fast rules for what is permissible when applying for planning permission, most Local Planning Authorities (LPAs) will have their own design guidance. These might include supplementary planning guidance (SPG), supplementary planning documents (SPD) or conservation area design guides. These documents are available on the relevant local authority websites and form a material consideration when determining an application. For example, Hackney – Residential extensions and alterations SPD.

 Where planning permission is required, it is common to use permitted development rules for scale and massing of an extension as guidance on what might be achievable. However, it is not always necessary to limit yourself to these rules. In our experience, given a strong argument and clearly communicated scheme, it is possible to improve on the massing of PD. On more sensitive sites, the LPA will resist PD equivalent applications. Learn more here.

House extension plans and examples

There is no one size fits all approach to extending your home. Ultimately all buildings are different (unless of course you live in an identical terraced or semi-detached property) and every extension is unique.

When planning your project, it is always useful to seek out examples of house extensions that appeal to you. This will help you become accustomed to reading 2D floor plans and understanding the types of spaces you want to achieve.

Note: The best place to look for example projects would be your local authority planning register. For example, if you live in Islington, you would look here. On the planning register you’ll be able to find details (including floor plans, elevations etc) of all applications that have been submitted in your area.

Here are a few examples of extensions that we have designed and gained planning permission for:

1930s Victorian London semi-detached house extension and loft conversion. Read More.

Georgian London terraced house two storey extension in Islington. Read more.

Georgian London terraced house two storey rear extension in the Barnsbury Conservation Area. Read more.

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