How much does a double story extension cost?

What is a two storey extension?

A double-height extension is a build to any aspect of your property that elevates up to provide two floors of space. This can be built to two storeys high to match your existing home, or as a side wing on a taller property.

One of the ideal solutions for maximal space in urban areas, two storey extensions provide double the living space, whilst using a proportionally smaller area of your land to build upon.

Another benefit is that the living space of your home is evenly distributed across its foundations. Some single storey conversions provide a large amount of living space on the ground floor with insufficient sleeping areas upstairs. This is a limiting factor on the saleable value of your property, and so building up to double-height helps balance out the interior.

Why do homeowners choose two storey extensions?

Choosing to build an extension is an alternative to moving home if you require more living space, and either cannot find a property you would like to move to or wish to avoid selling your home and having the stress of a relocation.

This can be an excellent proposition for properties in lower market value areas, as not only will your house increase in value, but also you can avoid losing equity by selling up and relocating elsewhere.

In terms of budgets, extending up over two floors costs little more than extending to one storey. This is because one of the most expensive factors of the build is creating the foundations, and exactly the same amount of work and materials are required for two floors as there is for one.

Whilst the cost per square metre is significantly less than for a single floor extension, the increase in market value to your home could be large. The average extension of this size increases the UK sale price of your property by anything from 10% and upwards.

In London, properties with double-height extensions can cost towards the higher end of the scale, but a cost of £2,800 per m2 will render an increase in the value of your home by around £4,000 per m2.

How much does a two storey extension cost?

The cost of your extension will depend on factors specific to your home and choice of design, including:

• Where your property is based – works in London and the southeast will always cost more than further north.
• How easy it is to access the site – if you live in a built-up urban area, specialist equipment may be required to access the site.
• What sort of soil your property has – some types of soil require different treatments and materials to create robust foundations.
• How large your extension is, both in footprint and in square meterage.
• What shape your roof design is, and what materials it is made from.
• Which finishes you choose – including glazing, flooring window and doorframes, roof lights and underfloor heating.

What types of two storey extensions are there?

Your extension design will depend on the layout and construction of your existing home. Typical types of double-height conversions include:

Two storey rear extensions
This type of conversion builds an additional two floors at the back of your property, usually building on garden space. This is best suited for detached or semi-detached homes where there is available space behind the house to extend onto.

Two storey side return conversions
A side conversion builds to either side of a home, without eating into the outdoor space. This is a very popular choice with period properties that have redundant space running alongside the home that doesn’t serve any other useful purpose.

Two storey wrap around extensions
A wrap around extension combines both previous options; it builds to the rear of the property, and also along the side to create a much larger floor space. This is the maximal way to increase the size of your property but is also one of the more expensive options.

Two storey front extensions
Although rear extensions are far more common, it is also possible to extend outwards from the front of your home. This can make a significant change to the curb appeal of your home, particularly if the existing design is dated. Whilst the planning rules are stricter since your conversion will likely face the road, a front extension is still possible.

Two storey extension planning costs explained

The initial costs in planning and designing a double-height extension include professional fees such as:

• Planning application costs
• Building certification costs
• Professional design costs
• Party wall agreement costs
• Other consultancy fees

It is worth shopping around for the most competitive rates and asking for recommendations to use the best professionals and avoid paying over the odds.

Planning permission for two storey extensions

Some properties may not require planning permission, but if you do this currently costs around £220 and takes around 8-13 weeks to complete. If in any doubt, contact your local planning authority who will advise, for a small fee, whether you are required to make an application.

Even if you can build without planning, you should consider applying to your council for a Certificate of Lawfulness, which costs around £100.

Professional design fees for a two storey conversion

For a large conversion, it is usually necessary to appoint an architect or designer to plan the layout and construction of your extension.

Typically, architects fees cost between 3-7% of the total construction cost and as high as 15% in London. However, using a local architect with knowledge of local regulations, materials and building constructions can pay dividends in the long run.

A double extension will require technical drawings to comply with building regulations, which tend to cost from £3,000 and upwards.

Another set of designs will be required for structural planning, and the cost for a structural property design can cost anything from £900 to £2,000 and higher for a complex design.

Party wall agreements for double-height extensions

Party walls are those property boundaries that run between your home and your neighbouring houses. You may have no party walls or may have them on each aspect of your property.

Should your extension build close to one of these boundaries, including excavation works, then you must seek a party wall agreement with those neighbours before work begins.

If you are happy to agree between yourselves on the work being carried out, then although you need to confirm this in writing it can be a fairly simple process.

You might, however, need to appoint a party wall surveyor to draw up a formal agreement, which can cost from £750 to £1,500 per household.

It is usually the applicant who wishes to build that is expected to cover these costs, so if you have more than one party wall it is important to budget for this expense.

Building Control requirements for two storey conversions

Building Control visit build sites at specified intervals to check that the construction is sound, is safe, and complies with building regulations.

They will issue certification to confirm that the build is compliant, which costs around £750 – although may be higher if any issues arise or the build is particularly unusual.

Professional fees involved in a two storey extension

Other fees to consider include:

• Building surveys – if you require, or are required to have, your existing property surveyed before work can start.
• Tree reports – particularly if you are in a TPO area, you will need reports to establish whether trees can be moved, cut back or replaced.
• Flood risk assessments – if you are in a high-risk flood area, you may need to commission a flood risk report.
• Ecology reports – if you live close to, or within a conservation area, park, area of grassland or other natural resources you might be required to have an ecology report produced assessing the impact of your build.
• Listed buildings – if you live in a listed building or conservation area, you will require listed building consent before you may begin work.

Frequently asked questions

How long does a two storey extension take to plan?

The duration of your build project will depend on how simple or complicated the structure is, how much internals works are required and whether there are any delays with planning permission and getting your party wall negotiations completed.

Typically, the project phases work as below:

1. Planning Stage: 16 weeks – 20 weeks
The design process will usually take around 8 weeks. You should sit down with your designer, explain what you have in mind, and show them any images of similar developments that have inspired you.

They will also need to visit the site, and take all the appropriate measurements to be able to draw up a thorough design.

Once you have a finished design, you will need to undergo the planning application process – if applicable. This can take up to 3 months should any amendments or revisions be required to your design before permission is granted.

2. Preparation Stage: 6 weeks – 6 months
The party wall stage is the biggest variable since although it is possible to have an agreement in place within a couple of weeks this can take several months if your neighbours object to the build.

Once you have an agreement, the technical drawings will be drawn up from the approved design. This includes drawings to comply with building regulations to demonstrate the structural design and typically takes between 4 and 8 weeks.

3. Construction Stage: 3 – 4 months
Very complex or large conversions may take longer, but most construction projects take around 14-16 weeks. These stages are set out below.

What are the build stages of a two storey extension?

It may sound surprising, but the build itself is very likely to take less time than the preparation, planning and design.

This is because, by the time the build starts, all of the planning and intricate calculations have been performed, and it is a case of ordering the materials, having them delivered to site, and constructing your new conversion.

A build process usually follows this general timescale:

• Stage 1: Excavation

Initially, the ground to be built on must be excavated, and the foundations laid. This can take as little as one week and upwards of three weeks depending on the soil, depth of foundations and how easy site access is.

• Stage 2: Structural beams

Once the ground is prepared and foundations are ready to be built on, the structural beams and columns will be erected. This takes around one week.

• Stage 3: External walls

Now your external walls can be constructed to form the outer layer of your conversion and will take around two weeks to build.

• Stage 4: Ground floor

Once the walls are ready, the slab can be laid on the ground floor. The suspended timbers to build the second floor on will also be built, so you can start to see your upper level taking shape!

• Stage 5: Roofing

Now your roof can be built. This includes erecting the timbers and framing, waterproofing and laying your selected roofing material. Typically, erecting and finishing a roof will take two weeks.

• Stage 6: Plastering

At this stage, the outline of the extension will be complete, and the building will be waterproofed to ensure it stays dry until the glazing is ready to be fitted. Your contractor will now start work on dry lining your extension and plastering internally, which will take around two weeks.

• Stage 7: Electrics

Once the plaster is dry, your electrics and plumbing will start to take shape. Often this involves two stages of fit-outs, whereby your tradespeople will initially run pipes and cables into the extension, and then complete power points and taps once the interior decorating is ready to start.

• Stage 8: Decoration

Your decorating can now start, including painting. This will take around one week, although may be delayed if any parts of the plaster are yet to dry thoroughly; this is more common in winter builds when there is more condensation.

• Stage 9: Flooring

The final touches are completed at this stage, with flooring fitting, skirting boards, windows and doors installed, and any other joinery work taking place. This work tends to take around two weeks.

Some contractors will install window frames and doors alongside the external walls, usually once the roofing is erected, and others will choose to weatherproof the conversion to preserve the glass and avoid any chance of damage whilst work is ongoing.

Don’t forget to complete the snagging stage! This involves checking every aspect of your conversion, checking doors and windows open and sit flush, and ensuring every utility works correctly – at this stage, you can sign off the build and pay the final balance.

Do I need planning permission for a two storey extension?

Whether or not you need planning permission depends on the nature of your extension, where it sits in relation to your home, and how large it is.

As an outline, the criteria that a conversion must meet to be eligible for Permitted Development (PD) that does not require planning permission, are:

1. The extension must not constitute more than 50% of the total free land around your property – bear in mind that if any previous extensions have been built, they will contribute to this total.
2. Your conversion blends in with the surrounding homes, or with your existing property.
3. The width of your extension is not more than 50% of the width of your home.
4. The length of your conversion is not longer than 3 metres for terraced or semi-detached homes, and not longer than 4 metres for detached houses.
5. For a double-height extension, the maximum height must not be higher than the eaves on your existing home.
6. Your design does not include any terraces or balconies.

It is more likely that you will require permission for a double-height extension than for a single storey conversion, so unless you have been advised that your design is admissible under PD, you should factor this time and cost into your plans.

Exclusions to permitted development

If you are in a conservation area or live in a listed building, you will always require permission before works begin, and cannot assume PD.

There are other exclusions that you should be aware of, in which case planning permission is always required.

• Your property must be a house – PD does not apply to any other type of property, including flats and maisonettes.
• Your property has not been converted to a home – If your home used to be a non-residential building and was converted to a house with a change of use permission, PD may not apply.
• Your house is not a listed building – All listed buildings and properties adjacent to listed buildings within the same boundary will require listed building consent. Be prepared for a more in-depth application process, which will take longer than a standard planning permission application.
• Your home is not in a conservation area – Any residences in conservation areas or specifically designed land are not able to assume PD. This includes any homes on heritage sites or within areas of outstanding natural beauty.

How much will it cost to fit out my extension?

Much of the fit-out costs depend on the intended use of your extension. Needless to say, a new kitchen or bathroom will cost more than a utility room or living area.

As a guide, the typical cost of painting and decorating is around £40-£60 per m2, although this increases if you choose a specialist type of paint.

Flooring usually costs between £40-£150 per m2, again depending on what type of flooring you choose. Specialist finishes like polished concrete can cost significantly more than carpets, laminate flooring, tiles or vinyl.

Heating costs depend on how well insulated your extension it and usually sit at around £60-£100 per m2. Underfloor heating is increasingly popular but is by far the most expensive option.

If you need to change your boiler to work within the new layout, or to have enough capacity to cope with the increased side of your home, you will need to budget for this cost. A new boiler can cost anything from £2,000-£3,500 for a standard residential boiler and potentially more depending on your requirements.

Fitting a new kitchen can cost anywhere from £5,000 for a simple, smaller kitchen, and for a higher quality or larger kitchen prices start from £15,000 including fitting and plumbing.

If you choose Bifold doors for your new downstairs, these can cost between £1,500-£3,000 per linear metre.

Should you be planning on a new bathroom for upstairs, the minimum budget would start from £3,500. As with kitchens, this depends on what sort of units you choose, and what tiles and appliances you wish to install.

Will building a double extension increase my house value?

Expanding your home by two floors, with both upper and lower rooms will usually add around double the cost of the conversion to your home, per m2.

For example, a London extension will cost up to £2,800 per m2, but on average will add £4,000 per m2 to the value of your home.

A double-storey extension is the most cost-effective conversion project in terms of cost per m2, and will almost always cost less than moving to a larger home.

If one of the primary reasons for choosing to extend your home is to increase the future value of your property, it could be worth appointing an independent valuer to help you understand the anticipated impact on value. This will help you to calculate whether your budget is viable, and what sort of return on your investment you can expect in today’s housing market.