How Do I Put An Extension On My House?

If you’re reluctant to move because you love your house and where it is, but you need some extra square footage, then a house extension can transform the space you do have to give you what you need.

Whether you intend to extend up, out or to the side, there are some key things to know before your project begins, from the legalities and logistics to budgets and builders.

Extending your home is a popular way to increase space and add value to your property. With the hassle and costs of moving house – from legal fees to stamp duty – the reasons to stay put and improve your existing home soon mount up. But where do you start?

Whether you’re thinking of a garage extension, side extension, double or single-storey rear extension, there is much to consider as you embark on an improvement project to expand your home. To help get you started, we take it from the top with pointers on planning rules, building regulations, through to handling the neighbours and finding a builder.

Adding an extension to your property is an effective way of creating more living space, perhaps to give current occupants more room, or to accommodate a growing family or an elderly relative. It is often a more economical option than moving house, not least because buying a new property incurs stamp duty and estate agent fees – which in some cases might amount to the cost of an extension itself.

Below is a step-by-step guide to the main challenges you will face as you advance from deciding to build an extension in the first instance, to moving into the final structure.

What Is The First Step?

No matter how big or small, every extension project requires a custom design solution. It is an opportunity to improve the liveability of a house and a chance to add value to the property. Therefore, it is critical that you get professional help at the design stage of the project to ensure you get the best possible outcome for your given budget. This should be your first step.

The options in most cases are to engage a draftsperson, a building designer or an architect, and generally speaking, the fees associated will increase in that order. However, rather than focus on the exact profession of who you might engage, I would suggest you get familiar with the type of work they do. Ask friends and colleagues for recommendations, and when interviewing, you’re shortlisted of preferred designers, ensure they are able to demonstrate experience in working with a brief and a budget that is similar to yours. Planning for a new look for your house? Look no further!  Home Builders  is here to help in your dual occupancy builder Melbourne.

Do House Extensions Add Value?

For house extensions to make economic sense, you need to make sure the value-added is greater than the cost of the project. It can be not easy to assess, but finding similar local properties and seeing how much they’ve sold for can be a useful guide.

Be aware of the ceiling value in your area, and be prepared to adjust your plans if necessary.

House Extensions Need to Comply With Building Regs 

Even if your house extension can be built under Permitted Development rights, work must get Building Regulations approval.

The Building Regulations set out minimum requirements for:

  • structural integrity
  • fire safety
  • energy efficiency
  • damp proofing
  • ventilation
  • and other key aspects that ensure a building is safe.

Most repair work is excluded from Building Regulations, with the exceptions of replacement windows, under­­pinning and rewiring. However, apart from certain new buildings such as sheds, outbuildings and some conser­vatories, all new building work, including altera­tions, must comply with the Regulations.

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Typical Examples of Work Needing Approval:

  • House extensions 
  • Loft conversions
  • Internal structural alterations, such as knocking down an internal wall that is loadbearing
  • Installation of baths, showers, WCs which involve new drainage or waste plumbing
  • Installation of new heating appliances
  • New chimneys or flues
  • Altered openings for new windows

Should I Build As Big A House Extension As Possible?

Often extenders get preoccupied with only thinking of the project in terms of square metres, not in terms of what that size is adding to the house. Bigger is not always better when it comes to house extensions, and there are often ways of creating the feeling of more space, without adding a large extension.

This is often achieved through clever design, not only of the new space but also what is already there.

Choose the Right Designer for Your Home Extension

When it comes to the design of your extensions, there are a number of options you can choose from.

These include:

  • architects
  • architectural technicians
  • specialist designers
  • package build companies’ in-house design teams

Ask for recommendations from friends, family and neighbours, but also look online for practices that have designed projects similar to what you are hoping to build.

Design In Efficiency Early On

By focusing on the fabric of your new house extension, you may be able to outstrip far the U values and airtightness levels specified by the Building Regulations.

However, appending a thermally-efficient extension to a poorly insulated home will not make it cheaper to run overnight, and you should look to improve the efficiency of the main house while the builders are on site.

Building A House Extension On Or Near A Sewer

If your house extension is built over or in the area of a sewer, you will need to contact your water board before work begins. “The location of sewers needs to be carefully considered. If a shared sewer (one which serves more than one property) is within 3m of your extension, then a Build Over Agreement with your local water authority is likely to be required.”

These can be tricky – and costly – especially if a new manhole is needed, or an existing one needs to be moved. Home Builders has the best range of dual occupancy builder services to help you create your dream house.

Extending Above A Single Storey

While it may seem appealing to extend above a single-storey extension or garage, these structures may not be able to support the load.

There are options where the old structure isn’t up to scratch: underpin existing shallow foundations; strengthen or bypass the existing with a steel frame bedded in new concrete pad footings, or demolish and rebuild. The latter is often the most cost-effective option.

Planning In Services For A House Extension

If you are extending your kitchen, you need to confirm the position of your units, cooker and white goods before work begins so that electrics, ventilation and plumbing can be planned in.

The same will apply if you’re building a two-storey extension and are including a new bathroom or en suite in the design.

Can Your Boiler Cope With The House Extension?

Adding house extensions will add demand to current hot water systems, which may not be able to cope.

It’s advised that you work out what the new extension will need to be heated effectively, and factor in your boiler output, the size of the radiators, hot water cylinder size and the reheat time.

Living On-Site Could Slow Down Progress

It is possible to live on-site throughout a house extension, but aside from all the dust and mess, you may end up slowing down progress as the builders attempt to work around your life.

If you’re not prepared to live with the disruption, then you should consider looking for temporary accommodation (short-term rental, hotel or staying with family or friends).

Factor In Access Restrictions

If you live in a terraced home with restricted access, that may affect the options you have for your house extension design. For example, you may not be able to use certain construction methods, or you may need to make arrangements with your neighbours to temporarily remove fence panels or use their land for short-term storage.

Connecting The Old And The New

How well the additional space sits alongside the original property will undoubtedly affect the success of the project. While there are no hard and fast rules, you will need to make a decision on whether you want your new house extension to complement or contrast with the main house.

Designing An Extension

Finding an architect

Depending on the scale of the project, you may wish to involve an architect. There’s no law saying you need to use an architect, even for large renovations and builds, but some people find it easier to have a professional draw up the designs. Using an architect will generally mean you’ll get a better result; however, their fees are usually around 15%, so you’ll need to factor this into your budget.

How to brief an architect

When you meet your architect, it’s important you give as much detail as possible on what you want from the project, your timings, how and when you intend to pay them and what penalties will be in place if deadlines are missed. The architect will then send you a detailed appointment letter along with a contract to sign.

Obtaining structural engineer calculations

A structural engineer can also provide technical drawings and calculations which can be used to seek Building Regulation approval and will then be used by your building contractor and architect during the renovation work.

How Many Glazings Can I Add to My House Extension?

“Part L of the Building Regulations limits the total area of glazed elements in an extension to a maximum of 25% of the extension’s floor area.

“Particularly on small extensions, this poses a problem. A small kitchen extension, perhaps adding 20m² of space, could easily have its allowance taken up by a set of bi-folds – 4m(W) x 1.8m(H) = 7.2m² or 36% of the floor area – and will therefore be rejected by building control.

“There are several ways around this. First of all, you need to deduct the total area of the windows and doors that are being lost as part of the extension from your additional total. If that doesn’t get you below 25%, then you’ll need to show that the new glazed extension can meet the energy performance standards achieved by a non-glazed extension in other ways.

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“If that still fails, then you’ll need to commission an SAP assessment to show that the CO2 emission rate from your glazed extension would be no greater than the emission from a fully compliant extension of the same size.”

What Do I Need To Order And When? 

Your contractor should take care of ordering most of the materials you’ll need for the actual building work, e.g. bricks, plasterboard etc. but you’ll probably want to take care of some things yourself: 

  • Kitchens and bathrooms often have to be ordered weeks if not months before their installation date, so if you’re set on a certain thing in this respect check the lead times and plan accordingly, making sure that anything you order is in your name and not the builders. 
  • Rooflights and windows similarly can have lead times from 8 – 10 weeks. 
  • It can be tempting to get straight into ordering new furnishings and finishing touches for your dream space but try to hold off until you need them as not only will this put an unnecessary strain on your cash flow, you’ll have to find space for them in an already disrupted and cramped household until the dust has settled. The finish is complete on your extension. 
  • The exception on this is bespoke or made to order items which sometimes have long lead times and will need to be planned for, so you don’t end up sitting on the floor whilst you wait for your beautiful new sofa to arrive. 

Tradespeople And Installers 

In addition to your main building contractor, you may choose to hire independent tradespeople to complete different aspects of the work, for example, if you know a particularly brilliant plumber already or you want your windows and roof lights installed by a specialist. 

It is important to communicate your desires upfront to your main contractor up front and do the leg work in coordinating their visits to the site if your main contractor is unwilling to take on this extra responsibility for people they may not know or trust. 


Interior Design 

Once the dust has settled, and the site has been signed off, you can get into what a lot of people think of as the fun stuff. Whether you choose to go it alone or to hire an interior designer to help you plan the look of your new space, you’ll want to complete the planning well in advance of your planned completion date. This will allow you time to carefully select your colours and furnishings and allow for any lead time on made to order items. You probably want to hold off buying smaller items and finishing touches until the interior is mostly finished, to allow you to see what the main elements of the new room/s look like in the flesh. Bear in mind that you’ll have a long time to perfect the look of your new space, whereas wasted money on hasty purchases is less easy to recoup. 

Can We Get More Sunlight And Natural Warmth?

Older homes aren’t generally known for their solar passive design qualities, so one of the key considerations when extending is to investigate ways to get more sunlight and warmth into the house.

Understanding where north will give you a good idea of where the sun will be at different times of the day; and if you have already lived in the home for a while, you will know where the sun will enter the property from season to season. Capturing the northern sun should be one of your key priorities when extending, especially if you are adding a living room or family room that you spend a lot of time in. More natural sunlight will make it a much more comfortable space to live in, and will also reduce the amount of artificial heating required.

However, when adding larger windows, you must also be very aware of how exposed the windows are to summer sunlight, especially the west-facing afternoon sun. While having sunlight beaming into the space in winter is a lovely outcome, the opposite can be said for scorching hot summer sun as it can make the space unliveable. This is where a thorough understanding of the orientation, appropriate glazing specification and good use of shading elements is critical. Once again, a good architect or building designer will help achieve the best results in your circumstances. Finding the right duplex build is an important decision. Check out our range of the best home design constructions at Home Builders. 

Would We Need To Move Out?

The financial benefits of living in the house throughout an extension are obvious, not to mention the convenience of not having to uproot your life and routine. Unfortunately, it may not be that easy. Any extension project that also involves a significant amount of renovation work to the existing part of the house will be very difficult to live in throughout the build. In these circumstances, the project may be able to be staged so that you can move from one part of the house to the next.

But you must also be aware that staging a building project will generally mean it takes longer to build, which also means it will cost a little bit more.

How Long Will It Take?

How long is a piece of string? A quick and simple extension might only take four to six weeks. Say, for example, you happen to be adding only a small amount of space under an existing roofline, and the full range of tradespeople aren’t required for the job. A similar timeline would apply if you were adding a kitchen or bathroom.

However, as most extensions generally do include a wet area – and assuming that the extension is of a decent size – the time frame is probably more like three to six months. Large extensions that also involve a lot of renovations to the existing house may take even longer; especially if they involve second storey additions. Six to nine months might be a more realistic time frame in that case.

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