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Things to Know Before Extending Your Home

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The ‘Right to Light’ Exists in Law


Your neighbour may attempt to block your plans to extend your home by claiming they have a legal right to light to one or more of their windows. There is such a thing as a legally established ‘right to light’, usually estab­­lished auto­matically ‘by prescription’ after 20 years, however, it is only relevant in limited circum­stances. A right to light is a type of easement, like a right of way, and overrides any planning permission you might have and your permitted develop­ment rights. It can in theory, therefore, prevent you from blocking out a neighbour’s window. How­ever, it only provides for whatever light is reasonably required for the use of the building – it does not mean that your exten­sion cannot obstruct a neighbour’s window or view, or reduce the amount of sunlight entering – these are planning consid­er­ations. Rights of light are only likely to be relevant in city centres where buildings are very close together. In such circumstances contact a specialist lawyer.


You Can Have WCs Anywhere


It used to be a requirement of the Building Regulations for there to be a lobby between a WC and any other room. This was commonly conceived to be for reasons of hygiene and to relate to the kitchen where food is prepared. Although it is desirable for reasons of privacy to locate WC facilities off hallways, circulation space, lobbies or the utility room, this is no longer necessary under the Building Regula­tions. It is, however, necessary for there to be a wash basin and suitable ventilation to all WCs.


Minimum Ceiling Heights


Although the legal minimum ceiling height has now been removed from the Building Regulations, there is still a practical minimum height and this is especially worth thinking about in attic and cellar conversions. All rooms should normally have a floor to ceiling height of at least 2.1m throughout (standard ceiling height is 2.4m). In rooms with sloping ceilings, at least 50% of the floor area should normally have a floor to ceiling height of at least 2.1m.


Avoid Through Rooms


Working out the most efficient and practical way to access an extension is often the great­est design challenge. What is not acceptable is to sacrifice more than you are gaining, for instance by slicing up a good sized bedroom in order to gain access to an extension that adds only one more bedroom of a similar size. Using an existing room to access an extension rarely works unless it is sufficiently large and the furniture carefully arranged. Such rooms usually end up as nothing more than a corridor and a dumping ground for home­less storage.


Circulation space is very important to the healthy function of a house, ensuring that it is liveable and that the best use is made out of all of the space. When extending, this may mean rethinking the function of every room in order that the principle rooms, most importantly the kitchen, dining and living space, can all be accessed directly from the main hallway. In smaller houses where there is no space for a separate hallway, it is a good idea to have at least a small lobby or enclosed porch to create privacy from the front door.


Make Your Conservatory Part of Your Home


You can integrate a conservatory into the existing house to make it an extension to an existing room, rather than a bolt on, but you have to be careful with the design.


The Building Regulations require most conservatories to be separated from the existing house by exterior quality doors. Such a doorway with a threshold can leave the new space feeling isolated from the rest of the house and unless the conservatory is large enough to work as a room in its own right (the minimum is around 3m x 4m) it can end up being an expensive, underused space.


Double doors can be left open to help make a conservatory feel like part of the house, but even double glazed conservatories lose heat quickly and so most people end up closing their conservatory off for the winter months to help keep their home warm and their fuel bills down.


With a bit of redesign to reduce the glazed area, the section of exterior wall separating it from the existing house can be completely removed. This turns the conservatory into a true extension, and by incorporating sections of plastered wall and insulated solid roof, a conservatory can be used to extend an existing room such as a dining room or kitchen. To turn a conservatory into an extension you must provide your local building control department with calculations that show the amount of glazing in the windows, doors and roof of the conservatory/extension, together with the amount of glazing in the windows, doors and any rooflights in the original house, do not exceed 25% of the floor area of the conservatory and all floors of the house added together. New windows and doors in the conservatory/extension will need to meet the current U-values required by the Building Regulations. An alternative is to show that the glazed area of the windows, doors and roof of the conservatory does not exceed 25% of the floor area. However, this is generally far harder to achieve.


Consider Adding Basement Space


If you have an existing cellar, you can convert it into living space without using up your per­mitted develop­ment rights — the volume you can add to a building without needing planning perm­ission. Creating basement windows and external access will not usually require planning per­mis­sion either, although it is always worth checking your local authority’s policy on basements. All work must, however, comply with the Building Regula­tions laid out in the Approved Document – Basements for Dwellings 2000 (


Converting existing cellar space to bring it up to habitable standards costs from £1,000-1,500/m² providing there is already enough headroom. Creating a new basement beneath an existing building to add extra space is also possible. The cost is £3,000-4,000/m². Due to the cost, it is usually only financially viable to retrospectively add a basement in high value areas such as Central London.


Balance Your Accommodation


Although there are no legal requirements to provide more than one bathroom, it is practical to have a least one full bathroom on the same floor as the main bedrooms and for larger households, at least one en suite bathroom — ideally to the master bedroom.


If you are extending to add extra bed­rooms, clearly creating the number of bed­rooms needed for the house­hold is the main priority, but this should, if at all possible, be balanced by an increase in the number of bath­rooms. Future buyers will expect at least one bathroom and a shower room on a four or five bedroom house and without this the value will be constrained. An additional bathroom on the ground floor of a house should be a last resort — it is functional but will add little if anything to the property’s value as bathrooms away from bedrooms are impractical – other than for those who work outdoors – and home­buyers do not like them.


Know the Building Regulations


Even if you do not need planning permission for your extension, because you are using permitted development rights, you must get building regulation 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 the 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 Building Regulations.


Typical Examples of Work Needing Approval:


Home extensions such as for a kitchen, bedroom, lounge, etc.

Loft conversions. Internal structural alterations, such as the removal of a load-bearing wall.

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.

You Can Start Within 48 Hours of Notifying Building Control


Once you have dealt with planning, if you are in a hurry to start building your exten­­sion, you can commence work immediately after giving the local authority building control depar­­tment 48 hours’ notice. You are required to submit a ‘Building Notice’ and the required fee.


Generally the Buildi­ng Notice method is more suitable for simple works where detailed draw­ings are not required, but it can be used for any project, with the exception of work to listed buildings. For most extensions it is best to make a Full Plans application. This involves submitting detailed drawings, specifications, calculations and a location plan for inspection by the local authority, together with the appli­ca­tion forms and appropriate fee. Building control has to respond within five weeks unless you agree to give them an extension to two months. A Full Plans submission allows any irregularities to be resolved before work commences. With a Building Notice, the building control officers can ask for proof of compliance at any stage, so it is essential to make sure they make all necessary inspec­tions and provide any structural calcula­tions when requested. When the project is com­pleted and inspect­ed by the local authority, a completion certifi­cate will be issued which will prove useful if the property is ever to be sold on. Application fees are set individually by each local author­ity.


Remove Your Walls


You don’t need walls to define rooms. Creating larger, more open spaces will help to make a prop­erly feel larger. The fewer walls you use, the more spa­cious and light a property will feel. You may worry about losing rooms but you do not need walls to create rooms. You can define separate rooms and functions by using all sorts of other features such as furniture, lighting, floor coverings, decoration, floor or ceiling levels, and informal room dividers such as kitchen island or peninsula units, fire­places, open shelving, island walls or even the stair­case.


If you are unsure whether it will work, visit show houses built by developers. Most now feature at least an open plan kitchen breakfast space instead of a separate kitchen and dining room. If you are worried about resale value — don’t be; the developers know exactly what will sell for the most money.


There are Different Rules For Conservation Areas


If you live in a Conservation Area your permit­ted development rights – extensions and alterations that do not require planning per­mis­sion – are restricted. Each local authority has its own policy for Conservation Areas but generally the basis of the policy is to prevent the loss of character of the Conservation Area. Consequently planning is required for the addition of dormer windows or any other change to the shape or height of the existing roof, including the addition of Velux rooflights if they face the highway. This means that attic conversions will almost always require planning permission in a Conservation Area.


Have You Really Thought Through Your Loft Conversion?


Before embarking on an expensive attic con­ver­sion think carefully about the cost relative to the amount of useful space that can be gained, and the impact on the existing accom­­modation. In order to comply with the Building Regulations to form an additional bedroom in the roof space, the floor may need strength­ening and the roof will need to have at least 150mm of insulation, plus a 50mm clear air gap (unless you are able to replace the existing roofing felt with a breathable mem­brane — something that is only possible if you are removing and relaying the roof tiles or slates, or use the new gener­ation of super-thin multi-layer foil based insula­tion products). The result of bringing the loft space up to habitable requirements is that the ceiling height will typically need to be lowered by 60-100mm and the floor level raised by at least 15-20mm. Both of these changes will reduce the amount of usable space with good headroom. If steel purlins or joists need to be added, too, this can further reduce the space.


If the usable space is limited, consider raising the roof height by rebuilding it, or even lowering the existing ceiling height below. Accessing a habitable loft conversion requires a permanent staircase and this will need landing space.


If you decide the cost of a full conversion to create habitable room cannot be justified because of space constraints, why not con­sid­er improving the loft space to create storage or other amenity space, accessed by a fold-away loft ladder or space-saver stair­case? This can be achieved at a signif­icantly lower cost than a full conversion. Attic con­ver­sions cost from £600/m² for a simple con­version, up to £1,500/m² for more compli­cated work.


Extending a Listed Building Can Be Problematic


Permitted development rights do not apply to listed buildings, so any extensions will need both planning and listed building consent. The design of any extensions to a listed building will need to be of a very high quality and it will usually be necessary to use the services of a specialist architect or surveyor. It will be essential to work closely with the local authority conser­vation officer. Each officer will have their own perspective on what will or will not alter the character or setting of a listed building. It is a criminal offence to alter a listed building, inside or out, without listed building consent.


Permitted Development is Not Always Straightforward


The following permitted development rights apply to any property that has not already been extended, providing it is not listed or has had its rights specifically restricted:


Any terraced house (including an end of terrace house or any house in a Conservation Area) you can add a maximum of 10% or up to 50m³ (whichever is the greater) to the volume of the original house without need­ing to apply for planning permission.

For any other kind of house outside of a Conservation Area you can add a maximum of 15% or up to 70m3 (whichever is greater) to the volume of the original house without needing to apply for planning permission.

In all cases, 115m3 is the maximum that can be added to the volume of the original house without planning permission.

However some local authorities apply the following restrictions:


If an extension to your house comes within 5m of any building or structure belonging to your house (such as a garden shed or a detached garage — even if built at the same time as the house), the volume of that building or structure can count against the allowance for extensions or additions.

Any building or structure which has been added to your property, which is more than 10m³ in volume and within 5m of your home is treated as an extension and may reduce the allowances for further extensions without planning permission.

Minimum Room Sizes


It can be tempting to try and squeeze the maximum accommodation out of a property by subdividing existing and new space into as many bedrooms as required, particularly if budget or the size of the extension permit­ted is restricted. However, there are minimum sizes beyond which rooms will not function. When considering applications for conver­sions, most local authorities have recom­men­ded minimum room sizes which planning applica­tions must conform to — although the sizes can be relaxed in some circumstances.


Function of room / Minimum Size

Double bedroom / 10.2-11.2m²

Single bedroom / 6.5-8.4m²

Living room / 13-13.2m²

Dining room / 9.5m²

Kitchen / 7.2-7.65m²

Kitchen/diner / 12m²

Lounge/diner / 15-18m²

Bathroom and WC / 3.6m²

Bathroom only / 2.8m² (or 1m2 of circ.)

WC only / 1.3m²

Hallways and landings / 900mm width


In addition, rooms must always have external windows, with the exception of non-habitable rooms such as kitchens, bathrooms, dining rooms and studies.


Know the Party Wall Act


Your neighbours cannot stop you from build­ing up to, or even on, the boundary between your properties, even if it requires access onto their land (providing you have planning permission to do so, and there are no restrictive covenants).


The Party Wall Act etc. 1996 allows you to carry out work on, or up to, your neighbours’ land and buildings, formalising the arrange­ments while also protecting everyone’s inter­ests. This is not a matter covered by planning or building control. If your extension involves building or digging foundations within 3m of the boundary, party wall or party wall struc­ture, or because you are digging foun­dations within 6m of a boundary, the work will require you to comply with the Party Wall Act and you will need a surveyor to act on your behalf. The Act does not apply in Scotland.


There is a Difference Between an Estimate and a Quotation


An estimate is normally a contractor’s guess as to what your extension will cost. An estimate given verbally, or in writing, is not legally binding and the final bill may exceed it. A quotation is a definite price. When deciding which builder to choose, always get written quotations from at least two firms, ideally ones that have been recommended to you.


The written quotes should itemise the work to be done, provide a breakdown of costs and a total, and state whether VAT is included. When you receive the bids, check whether there are any exclusions or caveats which might involve extra expense, and compare provisional sums for work such as foundations to make sure you are comparing like with like.


Beware of Removing Trees


Some trees are protected by Tree Preservation Orders (TPOs). Even if an extension does not require planning permission you cannot alter or even prune a tree that has a TPO on it without planning permission. All trees within a Conservation Area are protected by legislation and effectively have a TPO on them providing they have a trunk of diameter greater than 75mm. Altering a tree that is protected by a TPO is a criminal offence and can result in substantial fines.


A Shower Room Can Be Added Anywhere


A basic shower room need measure only 900mm wide by 1.8m with or 2.6m if it is to include a WC and basin. Well-lit, ventilated and fully tiled, it can be a luxury space. The cost of creating a shower room within a bedroom will be £2,500-3,000 providing the plumbing is within reasonable proximity.


You Probably Can’t Claim VAT Relief


Most extensions will be subject to VAT on labour and materials at the standard rate of 20%, especially if you use a contractor to undertake the work. If you use local trade­smen who are not VAT registered you can save the 20% VAT on their labour, but you will still have to pay VAT on materials at the standard rate.


Some extension projects are eligible for VAT relief, such as work to listed buildings (zero rated), the conversion of an existing dwelling that changes the number of units (reduced rate of 5%) and work to a building that has been unoccupied for at least two years (reduced rate of 5%). To benefit from VAT relief you must use a VAT registered builder — you cannot reclaim the VAT yourself.