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Understanding Self-Build Extension Costs
Step by Step
sel-build extension costs

Understanding Self-Build Extension Costs: Step by Step

In last month’s blog post, our construction experts broke down the self-build extension process, covering everything from planning permission to builder selection. One key matter, however, was left for later discussion.

Cost is one of the most vital factors of any self-build project. In order to ensure that your extension plans stay on track and within budget, it’s vital to develop a keen understanding of the financial elements involved prior to breaking ground.

Today’s follow-up focuses exclusively on the financial aspect of self-build extensions – providing a full overview of critical costing factors for consideration. Read on to help ensure the success of your project.

An Exploration of Self-Build Extension Costs By Stage

Our experts have broken the costs of self-build extension projects into five primary stages, listing factors to consider within each. With this being said, keep in mind that many builders and contractors will not separate pricing for these elements, combining, for example, materials and labour into one fixed cost. An overview of the UK’s collective average price points for this project type can be found at the end of this article.

An Exploration of Self-Build Extension Costs By Stage

Before you start building, it’s important to complete the extension planning process in full. This will help you set realistic expectations, identify your needs and wants, and ultimately, establish and stay within your budget. Most specialists recommend accounting for the following factors in the earliest phases of your project:

Architect Assistance and Drawings

Whether or not you’ll need to factor in the cost of an architect’s services ultimately depends on the scope and specifications of your project. Put simply, if your extension plans exceed permitted development allowances, architectural drawings will be required to obtain planning permission – and therefore must be budgeted for.

Most firms recommend taking the traditional approach to financial planning for drawings and designs – setting aside 5 to 15% of your ceiling cost for a thorough drawing package set to push your project through planning authoritys’ approval processes with relative ease. With this being said, some architects operate on a per-hour (averaging between £65 and £90), or fixed fee (between £2,000 and £6,000 in most cases) costing basis should this option be better suited to your circumstances.

Planning Permission

The planning permission process is designed to ensure that your final structure meets your local planning authority’s statutory definition of ‘development’ and complies with all applicable building policies.

As mentioned above, not all projects require such formal approvals. If your build plans fall within permitted development allowances, a simple architectural sketch or collaborative plan put together by your contractors may suffice.

Your local planning office should be able to provide vital guidance on this subject, specifying whether or not your project will require permission, and the type needed if so. For reference, in England, current fees for both full planning permission for alterations to a single dwelling house or flat and householder permission sit at £206, whilst outline permission rings in at £426 per 0.1 hectares of land.

Amendments to and resubmissions of an existing application, if needed, are typically free of charge.

Building Regulations Approval

Building regulations are a set of standards designed to ensure that all new construction projects are safe, structurally sound, energy-efficient, and do not pose any immediate detriment to their surroundings.

Your initial plans and drawings may include information regarding how the proposed extension will meet building regulations – however, this does not mean that approval will automatically be granted. In order to ensure the legality of your final construction, you must still make a separate application for review by your local building regulations department, even if you don’t require planning permission. 

The cost of submitting plans for building regulations approval can vary according to the type of application submitted (with both full applications and building notices on the cards for most extension projects), local authority fees, and project complexity – and is heavily influenced by the number of site inspections deemed to be required. Costs across the UK currently average out at £100 in application fees and a further £200 to £400 for inspections. 

For more information regarding precise building regulations fees, our team advise contacting your local authority or reaching out to a private sector-approved inspector.

Party Wall Agreements

If your self-build extension plans are attached or close to one or more shared walls, a party wall agreement will be required in order to begin work. This is a common scenario for those aiming to construct a full-width extension, for example, to the rear of their home. Should you be in this position, the M Jarrald team highly recommend liaising with your neighbours well in advance of the submission of an official notice in order to minimise the chance of disputes. 

If your party wall plans face rejection from neighbours, you’ll most likely have to employ the services of a party wall surveyor – a specialist trained to resolve contention of this type. Their fees can range from approximately £90 to £450 per hour, with average total costs coming in at £1,000 per project.

self-build extension costs
self-build extension costs


Once plans and upfront costs related to permissions are established, the next critical aspect to consider for a successful self-build extension is materials. These play a significant role in determining factors such as durability, aesthetics, and, of course, budget.

Whether your labour costs are non-inclusive of materials, or you’re working together with your contractor to establish the specifications of your project, the M Jarrald team recommend taking the following material costing factors into consideration:

Shell Material Types

Your extension’s shell materials, whether brick and mortar or timber, are a crucial factor in every aspect of its lifetime, from initial planning applications to overall longevity. With so many options available, it’s vital to understand their cost differences before making a final decision.

For example, whilst brick extensions pose a higher upfront cost than their timber-frame equivalents, their durability and weather resistance can make for a long-lasting, low-maintenance structure – particularly when subject to the UK’s dreary conditions.

Blockwork at its most basic (masonry only) is priced at approximately £10 per m² – or £12 when insulated to minimum standards. This method’s timber frame equivalent can see costs of up to £72 per m².

The amount of glazing required – determined in your initial plans – will also hold a significant influence over final figures.

Suppliers and Material Quality

Suppliers can hold a significant level of influence over material costs, making research and ‘shopping around’ important steps in any self-build project. For example, brick prices can vary from a few hundred pounds to over one thousand per thousand depending on both quality and the company you place your order with. Similarly, roofing materials can average out at anywhere between £50 and £150 per m² in line with the same factors.

Be sure to compare both before making final selections – your construction contractor may be able to offer both valuable industry contacts and general guidance at this point.

Delivery and Storage

Material costs don’t end at quantity, quality, and supplier. After making your selection, and working with your contractor to place your order, you’ll most likely need to factor in delivery costs and determine how the materials will be stored once they arrive on site.

Whilst some suppliers may offer free delivery, the vast majority, particularly when working with bulk or oversized orders, will charge a fee based on the distance from their facilities to your property – giving all the more reason to shop locally where possible.

In order to avoid the added costs of storage and wastage, you should ensure that either your deliveries are correctly timed for immediate use, or you have adequate storage space for your materials on-site.

Construction and Labour

Most self-builders go one of two routes when it comes to the building phase of their project – hiring individual tradespeople, or bringing on board a team of contractors. Regardless of which you choose, do so carefully – labour is one of the most important and significant expenses you’ll face from beginning to end, not to mention highly influential over the final quality of your extension.

Contractors and Tradespeople

Successful self-builds are most often supported by a professional, experienced team of tradespeople – whether hired on an individual basis for different aspects of the project or working as a collective. You may find yourself partnering with a large contracting team, or seeking a builder, electrician, plumber, and carpenter separately.

These people’s work will likely account for a significant portion of your budget. Teams with a good portfolio and extensive experience in particular will likely demand higher rates than those at the other end of the scale, however, are often worth the investment, offering industry expertise, and timely completions.

Labour can be charged for in three ways; fixed price (in which one figure is quoted for the entire job regardless of time and materials), per day, and per hour. Fixed prices, on average, will account for approximately 30% of your total project cost, whilst day rate averages sit at £200, and hourly at anywhere from £15 to £60 depending on the work undertaken.


In today’s disrupted marketplace, factoring contingency costs into your budget is a must. Whether resulting from broken supply chains and stock issues to unexpected on-site hitches, unforeseen costs can arise at any time. It’s essential to budget for these unwelcome surprises in order to ensure work on your project is able to continue smoothly – mitigating the impact of any unexpected expenses.

Our experts typically recommend adding at least 5 to 10% of contingency to your final budget.

self-build extension costs
self-build extension costs

Fitting and Finishings

Post-construction, attention should turn to finishing your project – taking it from an empty shell to part of your home. Whilst some contractors factor finishing into fixed quotes, those working on a stage-by-stage basis or hiring individual tradesmen for tasks such as installation and decoration must take added costs into account.

Finishing Quality and Installation

Estimating finishing costs in their entirety can be difficult, as they’re influenced by the price of both labour and materials. In addition, your extension’s size and complexity can play a role in financials at this stage. 

Whether you’re in need of painting and decorating (£400 per room on average) internally or cedar cladding installation (£165 per metre on average) externally, it’s vital to obtain quotes upfront.

Extension Features and Functionality

Are you planning on fitting a kitchen or bathroom within your extension? Be sure to budget accordingly. The costs of adding the necessary plumbing, wiring, and fixtures can be significant – particularly in complex scenarios. 

Most industry experts recommend allocating at least an extra £2,500 to £5,000 for a bathroom extension, whether this be a downstairs toilet or walk-in shower room. This should cover plumbing, materials, and fitting costs. 

Those looking to add a kitchen to their extension should expect costs associated with specialist fitting and fishing skills, alongside those of a new kitchen itself. With material prices for full sets of cabinets, worktops, and appliances coming in at anywhere from £10,000 to £25,000 plus in addition to plumbing, gas and electrical wiring (with installers commanding hourly rates of between £30 and £60 per service), a kitchen extension can amount to a large financial undertaking.

Other Costs and Factors to Consider

The primary costs involved in a self-build extension come in the form of the planning, material, labour, and finishing costs already covered – however, the M Jarrald team recommend taking the following additional factors into account when establishing your budget:

Site Preparation and Clearance

A good extension build needs a clean slate for construction. Before pouring the foundations for your project, you’ll need to ensure the site is cleared of any existing structures, trees, or debris. 

Your contractors may be able to undertake this job for you – including it within their daily or hourly rate or fixed price. Should they be unable to, you may opt to hire a professional site clearance company, or the necessary equipment and undertake the job yourself. 

In most cases, clearance costs for a small-scale space, such as a garden, average out at £300. Where demolition is required, this figure can increase significantly.

Home and Extension Insurance

Be sure to notify your home insurance company of any planned developments prior to the beginning of work. In most cases, your premium will increase as a result of the square footage an extension will add to your home (meaning it would cost more to rebuild in the event of destruction). Your contents insurance payments may also need to increase in line with any new furniture or finishings.

Most home insurance policies will ensure that your extension is at least partially covered for the duration of construction, and included in your policy once completed. Should you be seeking full coverage for the risks faced during the initial phases of the project, a temporary extension insurance plan may be the best option – with costs increasing in line with the level of protection required.


The price points of all of the above documentation, applications, professional fees, materials, and labour can fluctuate in accordance with regional variances. 

In general, the vast majority are higher in the South of England, particularly London and the surrounding areas, however, issues such as regional shortages and long transport distances can add to total costs in any location.

self-build extensions costs
self-build extension costs

How Much Do Self-Build Extensions Cost On Average?

As has been established above, the final cost of any self-build extension is influenced heavily by a wide range of factors – from initial specifications and style to fixtures and finishings. 

As an approximate guide, most basic single-storey extensions with minimal finishings start at £1,500 per m², whilst more complex or premium designs typically cost from £2,000 to upwards of £3,000 per m². This places broad average prices for a 20m² extension at between £30,000 and £60,000. 

Those planning two-storey extensions needn’t necessarily double these figures. As the project’s foundations and roof are already covered within the single storey built, walls, flooring, joists, and fixtures make up the vast majority of the added cost. Typical 60m² double storey extensions come in at around £105,000 to £155,000.

Our Tips for Staying on Budget

With so many factors to consider and options available, staying on budget for the duration of your self-build extension project can be challenging – however, with proper planning and preparation, it is possible to keep costs in line. Our experts recommend sticking to the following tips:

Prioritise Needs Over Wants

When faced with endless portfolios of high-spec extension builds and endless shiny finishing catalogues, it can be easy to get carried away. Making a conscious effort to distinguish between needs and wants at every stage of the process can assist greatly in avoiding overspending. 

Our team advise focusing on the essential features of your extension, establishing which purchases are necessary for its functionality and your living needs. To further this, ensure you’ve studied your local area’s ceiling prices prior to finalising your plans – there’s little point in going overboard on luxury finishes if they’re unable to add to your property value.

Compare Multiple Quotes

As mentioned above, tradespeople, contractors, and suppliers tend to set their own prices for materials and workmanship, quoting in line with their levels of quality and experience alongside factors such as distance and project complexity. 

In order to ensure you’re able to get the very best– whether dealing with bricks or building – obtain a variety of quotes before committing, comparing all of the above elements with the final figures seen on the invoice.

Avoid Mid-Project Changes

The most financially organised and smooth-running self-build projects work tightly in line with a pre-established plan. Any deviation from this can be both costly and time consuming, impacting all points of the process. For example, significant structural alterations may require changes to architectural drawings, and resubmissions of planning applications, whilst later adjustments may add days or weeks to labour, increasing costs.

Our experts recommend working closely with your initial planning team and contractor in order to establish a clear, detailed plan upfront. This will help ensure that the project stays on track, within budget, and that the final product meets your expectations.

How Can M Jarrald Help With Your House Extension?

M Jarrald Construction are composed of a highly skilled team of extension specialists – all of whom understand the importance of a well-budgeted project. We strive to deliver bespoke self-build solutions from start to finish, taking our clients’ visions from plans on paper to expertly finished builds with pride. 

If you’re planning to extend your home, we can help. Whether you’re looking for a quote, or have queries about any point of the process, M Jarrald will be happy to help. Request a callback, or email us at contact@mjarrald.co.uk to further discuss your plans. If you’d prefer to pop by in person, why not visit us at our Hadleigh-based office?