Trimble Architects

RIBA Architectural Services

Phone Number: 020 8538 9303

Mobile Number: 07785711635

Where Do I Start When Building An Extension

  • Posted by:
  • Admin
  • Tags:
  • Posted date:
  • 25-07-2022
Where Do I Start When Building An Extension

Where do I start when building an extension? We look at the process of building an extension as well as question to ask before you start the project.

Bob Trimble is a chartered architect registered with RIBA with 30 years experience in the industry. Trimble Architects work throughout Hounslow, Twickenham, Richmond, Kingston Upon Thames, Teddington and the surrounding areas of London.

Where Do I Need to Start?

Looking for a way to increase your space without the hassle of uprooting your life and moving property? Well, building a home extension could be your best choice. 

With so many extension variations, there will be something that suits your needs perfectly. Moving home can require a lengthy list of tasks, from changing your address on thousands of documents to packing up your entire life into boxes- not ideal when all you need is some extra space.

But where do you need to start? Well, the first step is to consider the space available for the build, the permissions required depending on the type of house, and where the extension will lay. There's a wide selection of extensions available, such as side extensions, garage conversions, conservatories or double/single-storey rear extensions.

Here at Trimble Architects, we aim to help guide you to the perfect extension, offering pointers on building regulations, planning rules, dealing with neighbours and finding a professional and reputable building company. 

What to consider before you start your extension

Most home extensions or improvements will positively affect your home's value, whether a small downstairs toilet extension or large garage conversion. If you're more cautious about spending money without a guaranteed return, speak to a reliable local estate agent. They will be able to assess the home's value as it is and determine what additional value if any, would be added to the home after the proposed extension is complete.

A professional assessing the potential added value from an extension can also help guide your budget throughout the project.If money is no object, then building a home extension doesn't necessarily need to add additional monetary value for future buyers; instead, it focuses on improving the lives of those currently living in the house.

Some extension projects can commence without the requirement of planning permission, thanks to your 'permitted development rights. These rules state that you can extend the rear wall of a detached house to the rear by 8m (single-storey) or 3m (double-storey). For semi-detached or terraced houses, the limit is 6m. Height restrictions also stated that single-storey extensions, including ridge and eaves, shouldn't extend higher than 4m.

The ridge should never exceed the height of the original structure. Double-storey extensions must be at least 7m away from the property line. However, some additional restrictions must be followed. For instance, an extension that will cover more than half the area of land around the original house requires planning permission. Furthermore, extensions should be built with similar materials to the existing structure's exterior, especially with Listed Buildings.

Suppose your home is in a Conservation Area or is a Listed Property. In that case, there are more legal requirements for building works, so ensure you obtain the correct listed building consent from the authoritative bodies. 

You can complete some extension projects without planning permission if they fit into the correct category. Your permitted development rights protect your right to modify your home without authoritative permission; to an extent. Whether you require planning permission or not, every renovation or construction project MUST abide by building regulations in the UK.

You must ensure that the workpeople completing the project can either self-certify their work or speak with local Building Control Officers to confirm the work meets all building regulations. Self-certifiable workers can be FENSA accredited window fitters, Gas Safe registered gas engineers, and NICEIC registered electricians and more. If the extension doesn't meet the requirements, then you may be requested to stop all construction or even remove the parts already built.

Once served with a notice, it becomes a criminal offence to leave problematic building up or continue work without updating permissions. Moreover, when planning to sell your home, this will raise issues for both the seller and buyer. 

Before initiating any building work, you must contact your home and contents provider to inform them of your planned extension. The re-build cost of your home is likely to increase after adding additional spaces, with Insurers considering this factor whilst pricing premiums.Whilst completing the extension project, your house is more likely to receive damage, which can void insurance if you've not informed the company.

The insurer will be able to advise you whether the new extension is covered under the existing policy or not and whether this will entail a premium increase. If, for some reason, your insurer is unable to cover the property after an extension, you must find a cover before the previous cover is cancelled. Any workpeople on-site should also have professional indemnity insurance to cover costs in the case of an emergency. 

If you own the freehold, you can do whatever you want within governmental boundaries. However, if you only have the leasehold of a property, you must check the details of your lease to determine your right to make alterations, most often, these must be approved by the freeholder.

It's beneficial to get in contact with the freeholder as soon as possible to reduce time spent waiting and so you're aware of the additional costs associated with the process. 

A disagreement often seen between neighbours relates to building or renovation projects. If the extension is significant and planning permissions are required, then the local planning authority will contact your neighbours for you to discuss the plans.

Although for the sake of a good neighbourly relationship it's a good idea to speak to them yourself and let them know about the plans, this will reduce disagreements and excuse some disruptions.

Usually, between terraced or semi-detached houses, you'll find a 'party wall'; this wall is a shared wall that divides the home space between two separate owners. This can also include garden walls built over boundary lines or excavations close to a neighbour's property.

An agreement must be met for excavations within three or six meters of the neighbouring property line, depending on the depth of the new foundations. Often with loft conversions or loft extensions, new steel supports are needed, new foundations need to be dug and/, or a damp proof course needs to be installed. These all require a Party Wall Agreement. 

The scale of your project will determine whether you need to hire a professional architect or not. No laws specify that an architect is required for building works, even with larger and complex projects.

Although it can prove very beneficial to use an architect as they can create professional design drawings with exact measurements, which can help other teams working on the project. Fees for architects generally sit around 15% of the project's total cost, but with an improved result, it can be worth the extra cost. Just make sure you consider the architect fees during the initial budgeting stages. 

When building a home extension, you'll want to work with the most reliable and trusted tradespeople to ensure high-quality finished products. An easy way to determine which builders are the best is to check online reviews, as these are honest and to-the-point reviews from previous customers.

You can easily compare the potential contractors online with information on their specialities, prices, previous work and covered area. To easily compare building companies check out websites like Checkatrade. 

Planning Your Extension

Factor in Extended Timescales

Owing to the recent events around the world (COVID, recession etc.), industries have been experiencing a dramatic increase in delays in production, transportation and stocking of goods. Material and labour markets are still volatile and will likely be for some years until the economy recovers. 


According to professionals in the industry, you can still receive quotes for building work, although this may take a tiny bit longer. However, even when using a reputable builder, you could be waiting up to a year, or more, for the actual building work to begin!

House location may also affect the wait times for the building to commence. If you're lucky, a cancellation may pop up. Other than the possible cancellations, be prepared to wait up to a year for construction on your home extension to begin.

 Build Time

The complexity and cost of the proposed build will affect the build time. It suggested that for small extensions, you should allow five to six months as the minimum build time, although this can be affected by industry challenges.

Most costs for extension build exceed expected budgets, so prioritise time to check your savings, budget correctly and tuck away spare financing. Prices may even rise between the first cost suggestion and the final price, so when getting a quote to, check their likelihood of increasing.

A way to handle costs is to hire a Quantity Surveyor (QS), no matter the project size, they can help manage costs. 


What does a Building Extension Cost

The bottom line is that whatever costs you expect will be met and likely exceeded. Current world difficulties, like COVID, have meant that labour and material wait times have dramatically increased, along with rising costs for everything.

Furthermore, the size and type of extension will determine the costs, along with the location of the build. Simple single-storey extensions may cost between £1,800-£2,300 per m².Another increase in costs can be blamed on the location you live in. Higher-value areas, like conservation sites or parts down South, have been affected by labour shortages, meaning you could easily spend £3,000 per m² for an extension. 

Set a Realistic Budget

It's easy for money to run away from you whilst completing building work. When you're creating a new space, you tend to get carried away with design ideas that you may desire but can cost an arm and leg, with little or no predicted returns. Before starting anything, you must check your finances and work out the size and type of extension you can afford.

It's averaged that most extension projects costs between £1,500 and £2,000 per m² (metre squared). When budgeting as well as considering the build costs, you should take into consideration the cost of certain design decisions such as tiling, flooring, units being instaled etc. The predicted costs of an extension are more often than not exceeded, so it's vital you have a savings fund you can fall back on.

Generally, a contingency of around 20% should be considered, as unforeseen costs can occur no matter how much planning there is. The contingency may increase if the house is old, listed or in an obscure location. 

Who Will Project Manage the Extension Build

There are various options when deciding who will forefront the project management, and guide you through the process. The choice may depend on your build route, budget and type of extension. For a project manager, you could hire either an Architect or a Design and Build Company, as well as complete the management yourself or with a mixed team. Below we break down your options, so you can make the best choice.

 Hire an architect

A popular choice for extensions is to hire an architect or professional designer to draw up realistic plans and ensure the project begins on a good foot. Here at Trimble Architects, we assist with full design services, help to appoint staff, and co-ordination other building specialists involved in the project. This early interference can ensure co-ordination across the various consultants and workers before the project is underway.

Moreover, the architect will help guide you through planning permissions and necessary applications. When construction begins, the project manager's responsibilities can be passed on from the architect to the main contractor. The project manager's role during construction relies on appointing, coordinating, and managing multiple construction packages and contractors. At this stage of development, the architect should assist with communication between the builder and client (you).

 Employ a Design & Build company

 If you'd find it easier to have all workers from one company, there are certain design and build companies that offer a mix of in-house professionals. A good design and build company will have employees able to complete each part of the process, minimising the need for external input.

However, with larger and more complex projects, you should consider hiring a trusted project manager. They can oversee every part of the process with a wealth of knowledge and experience, saving you a lot of time and stress in the long term.

 Project manage yourself

Managing a building project yourself can come with many benefits, but these are easily outweighed by the positives you would have attained working with a professional. However, in the case your budget is extremely tight, or maybe you have previous experience, no laws are stopping you from managing the project yourself. Just be wary of Listed buildings or protected areas needing more permissions.

By managing yourself, you will gain a greater deal of control over the project, which can prove very satisfying and personally rewarding. Although it is easier for costs and other problems to slip under the radar if fewer people have their eyes on the bottom line. 

 Split the project management

This is a great way to retain control whilst ensuring professionals are providing the best service. You can split the responsibilities between a contractor and yourself; for example, you may require a building company for the construction stages and then go on to manage the sub-contractors (plumbers, electricians, etc.) yourself. 

Each project is different and relies on different skill sets; by determining your skills, you can maximise them, along with professional expertise, to achieve a high-quality result within budget. 

Which build route will you choose? 

Build route refers to the way you choose to build your extension, with your selection likely tied to costs and your budget. Below we give the typical costs for differing build routes;

  • DIY: £300-£700/m2
  • Self-managed: £600-£1,400/m2
  • Main contractor: £1,000-£1,700/m2
  • Design and build: £1,200-£2,000/m2  

 Talk To A Builder

An easy way to grasp the feasibility of your proposed project is to talk to some trusted local builders, as they can offer an engineering and construction perspective.

Builders have a wealth of information regarding building work issues, meaning they can offer invaluable and experienced advice. It's useful to talk to a builder before plans are drawn up to confirm the feasibility of the project.

The builder can help with structural details, groundwork and more. This information can then be taken to an architect, who can draw up plans according to the suggestions given.

For houses in obscure locations, consider speaking to a planning consultant; they can help with feasibility examinations and site surveys. 

choose to build your extension

Can I Build an Extension Cheaply?

There are ways to reduce the overall costs of your new extension. A great way is using budget-friendly materials and a construction system. Concrete blockwork is a popular choice, as it's readily available and easy to work with.

Some methods like structural insulated panels are more expensive on initial purchase yet reduce labour costs. Moreover, the shape and design of your extension will impact the price, the more simple the design, the cheaper it will be. Similarly, a simple flat roof will cost less than a complex angled design with a lot of glass. 

 Architects Near Me


If you are looking for architectural services throughout Hounslow, Twickenham and the greater London area, then get in contact with our team today. We can offer advice and begin guiding you towards making the right decision.


Bob Trimble is a chartered architect registered with RIBA. Bob Trimble has 30 years of experience working with residential and commercial property projects. For 4 years, Bob Trimble has worked from his housing association and private architectural practice for clients throughout Hounslow, Twickenham, Richmond, Kingston Upon Thames, Teddington and the surrounding areas of London.