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Guide to extending or altering your home

Last Modified October 19, 2015


This information is designed to assist you through the initial stages to the completion of your building work. It should answer:

  • What is my first step now I’ve decided to start building work on my home?
  • What details do I need to prepare?
  • Who should I get to carry out the work?
  • How will I know the work is being carried out properly?

You can also find out more by reading our Supplementary Planning Guidance or you can contact Building Control.

Section 1 - The First Step

The reason for extending or altering your home is usually to provide more space or to make the most of the room you already have. When deciding upon the building work, ask yourself:

  • Do I really need it?
  • What are the alternatives?
  • What can I afford to spend?
  • Do I need permission?
  • What effect will this have on my neighbours?

Ask family, friends and colleagues if they’ve had similar work carried out, who prepared the plans and did the building work, and most importantly, if they had any issues with them.

Invite at least three builders to give you a free rough estimate, mentioning if applicable that you obtained their names through recommendations, and ask them to state how long their estimate will be valid.

Contact the names recommended to you for drawing the plans and obtain an estimate for preparing working drawings and obtaining relevant approvals.

If you are contemplating a loft conversion contact at least three specialist firms (ensure they will prepare plans and structural calculations) and obtain a free quotation.

Do not sign anything until you have considered all three quotes.

Remember to consider how long it will take for the work to be carried out.

Section 2 - Financial considerations

Now you have your provisional quotes for the plans and building work contact the Building Control team to find out Building Regulation charges and likely planning fees.

If you have a mortgage, contact your building society or bank to arrange to meet their loans adviser for a home improvement or secured loan. At this meeting, find out:

  •  What administrative costs they charge?
  •  Will they require their surveyor to inspect the work before releasing funds and will they charge for the surveyor’s visits?
  •  Will they release funds at various stages - if so, they may require proof that the stage has been completed.


You should avoid agreeing to the loan being released once the work is completed, as you will have to find the funds yourself to pay the builder the stage payments (i.e. when the foundations have been laid, when the roof is on, when the walls and ceilings have been plastered, and, on completion of the works). If they release the funds in advance don’t pay it all over to your builder, agree in advance with the builder on payment in stages.

Will they consider revised estimates for unforeseen occurrences? For example, where foundations have to be taken deeper due to nearby drains, bad ground, tree roots, or because existing walls require underpinning.

Contact Building Control to find out which, if any, may affect your project.

What documentary evidence will they require? For example, an invoice or bill from the builder, an invoice from us (for the inspection charge), and receipts for all payments made in advance of the loan, such as for the preparation of the plans, for the Building Regulation plan approval, or for Planning Permission.

Will you need to increase your building insurance payments?

What will be your new monthly repayment?

When will the repayments start?

Will they pay the remainder of the loan on obtaining a final certificate from Building Control?

Agree with the builder in advance on stage payments and a final retention, whereby you keep an agreed amount for at least six months.

You are advised not to start work until you have obtained Building Regulation Approval, and ensure the builder has a copy of the approved plans and to confirm the agreed estimate remains unaltered.

A rough estimate for a single storey extension of floor area of 20m² with additional alterations up to £1000 is:

  • To prepare plans and specifications may cost between £300 and £700
  • For Building Control’s Regulation fee, go to our guide to building regulation charges page.
  • Planning fee (if required) – check the current fee using the Planning Portal.
  • An estimate of the cost of building the extension to a plastered finish (not including light fittings, kitchen units, tiling, floor finishes, decorating, etc.) is £1000 per m².
  • Completion certificate issued for new applications and for the first time are free.

Section 3 - Preparation of plans

The agent should be instructed to provide plans, specifications and (in the case of structural alterations and loft conversions) structural calculations suitable for obtaining Building Regulation Approval and Planning Permission.

Also for the purposes of obtaining a quotation for lighting, power points, central heating extension, tiling, decorating, floor covering and kitchen units to be included in the drawings and specifications.

The agent will usually apply for the approvals/permissions/consents and obtain copies of the plans. Normally the agent will issue a bill when the plans are deposited with us - find out if there will be any other charges should amendment to the plans be required by us or for any additional copies.

The Building Regulation Plan fee and the Planning Application fee are payable by the owner with the application.


If you do not have plans prepared you may find the builder will add on extra costs - the drawings and specifications prepared will ensure a more rigid quotation.

 Section 4 - Obtaining consents

You may need the following consents:

  • Building Regulation Approval (an alternative is to deposit a Building Notice)
  • Planning Permission
  • Listed Building Consent / Conservation Area Consents
  • Tree Preservation Orders
  • Public Sewer Owners consent
  • Land Drainage Byelaws
  • Permission from the original developer - check the house deeds to see if there is a restrictive covenant to this effect
  • Agreement from the building society or bank holding your mortgage
  • Your obligations under the Party Wall Act 1996 - agreement from your neighbour if your work affects them.
  • All drains serving more than one property are deemed to be public sewers and require a build over agreement from Thames Water if building within 3m of the sewer.

Failure to obtain all the relevant consents may affect the sale or re-mortgaging of your property. 

Building Regulation approval

Approval is required for most work involving structural alterations, such as re-roofing, loft conversions, converting garages, extensions, electrical works, installation of new sanitary fittings, boilers, etc.  Also for internal alterations such as removing chimney breasts, forming an opening in a load bearing wall or external walls and for replacing existing windows and doors.

If in doubt contact the Building Control team

For minor works it is possible to deposit a Building Notice whereby plans do not always need to be produced (Building Control will advise you when a Building Notice is appropriate).

Building Control application forms

With the Building Regulations Full Plans application - a plan fee is payable that covers checking your plan for compliance with Building Regulations. It is usual for amendments to be made to the plan before an approval is issued.

The three builders who have quoted should be given a copy of the approved plan with any amendments shown for a final quotation, as some of the amendments can seriously alter the cost e.g. the foundations may have to go down deeper because of trees or drains. 

If you have committed yourself to a builder before receiving approval you may have no option but to pay substantial additional payments for the extra work caused by the amendments to the deposited plans.  

Common amendments that may lead to substantial extra costs include:

a. Trees and hedges within 30m of your extension - make sure your agent includes all trees significant or otherwise on the drawing.

b. Drains and sewers - make sure your agent accurately plots the drains/sewers and manholes and notes the depths - these may affect your foundations and the positioning of your extension, as you may not be allowed to build over the sewer or manhole.

c. Existing foundations to be exposed - where building over an existing ground floor extension. The existing foundations will have to be exposed in case the original foundations are inadequate - if so, they may need to be underpinned. Ask the builder to quote for a provisional sum to cover this.

d. Existing ground levels - If the ground level adjacent to the extension is higher, the work may need to incorporate retaining structure and additional damp proofing. If the adjacent ground level is lower, the proposed foundations may need to be taken deeper and a suspended floor system may need to be used.

e. Existing lintels or beams to be exposed - where you are building over existing structural openings with a first floor extension or a loft conversion - the existing internal or external lintels over windows, doors or a through lounge opening may need to be replaced - again ask for a provisional sum to be included in the estimate.

f. Existing drains under a proposed extension - when exposed these may be found to be leaking or need replacing - ask for a provisional sum to be included in the estimate.

g. Rainwater drainage - new rainwater from the roof cannot go to the foul system (even if the existing does) - a soakaway 5m away from the building may be requested.

h. A means of escape in case of fire is required for a two storey house having its loft converted - this will entail the existing doors be replaced with 1/2 hour fire resistant doors.

i. Inner room situation - it is no longer possible to provide an open plan design without alternative means of escape in case of fire being provided from the inner rooms.

j. All windows to new first floor rooms (excluding bathrooms and toilets) - or where windows are being replaced - are to be fitted with means of escape openings and means of escape hinges.

Planning permission

This may be required for work affecting the external appearance of a building.

An extension, dormer windows, replacement windows, new window openings, roof lights, new garages, converting garages to rooms, new out buildings, satellite dishes and new fencing may all require Planning Permission.

There is a facility for consulting in writing for a Determination as to whether or not Planning Permission is required - for further details contact Development Management section. If permission is required a plan fee will need to be deposited with the application.

For further information visit visit our planning permission section.

Listed Building Consent and conservation areas

If your house is on the Statutory List of Buildings of Special Architectural or Historic Interest you will normally require consent for any alteration affecting the internal or external appearance of the building.

If you are located within a conservation area stricter controls apply to what can be built without the need for Planning Permission.

For further details contact us

Tree Preservation Orders

This may be required if you propose to cut down or lop back a tree that has a Preservation Order on it.

For further information contact our tree officer.

Public Sewer owners consent

It is your duty to find out if there is a Public Sewer within 3m of the new building and consent of Thames Water Utilities Limited is required. Contact Thames Water Utilities on 0118 923 6683.

It is also not possible to build over or incorporate a manhole within the building if it serves a public sewer.

Land Drainage Byelaws

Under The Land Drainage Byelaws 1998 you are required to obtain consent from our Drainage Services section if you intend to build within 9m of a watercourse.

Permission from the original developer

There may be a restrictive covenant on the deeds of your house, which mean you need permission from the original developer for an extension to be built - if the house was built some time ago the developer may no longer be in existence but this should not be taken for granted - you can usually obtain insurance protection for this via a one off premium. If you have bought the house from us you will need to contact our legal team, or William Sutton or Aldwyck Housing Association depending upon where you live.

A fee may be charged for this.

The building society or bank holding your mortgage

May require permission (whether or not you are going to obtain a loan from them) for any alterations, as it may affect their interest in the property - they may charge a fee for consenting to the proposals.

Party Wall etc. Act 1996

It may be that the work could affect your neighbour’s buildings (e.g. by undermining foundations, disturbing party walls or boundary walls). If so, you are obliged to give notice in writing of the intended work to all relevant adjoining owners.

You may also need their permission to remove fences, to go on their land to build your wall. Your gutter may project over their land or you may want to use their extension wall to set a weathering flashing against - all of which will require their legal agreement in writing and possibly attached to the deeds of the properties to avoid problems from future owners.

You are advised in all cases to warn your neighbours of intended work. Be considerate to their fears, avoid building right up to the boundary line and ensure the builder always leaves the adjoining owner’s property clean and makes good any damage to their satisfaction.

Any fences needing replacing may not be covered by your loan nor quoted for by the builder. 

Section 5 - Choosing a builder

When you have received Building Regulation approval and (if needed) Planning Permission, go back to the three builders for a final quotation.

Obtain references from the builder, view their work, contact previous customers and take note of any complaints.

Find out if they are registered with any of the relevant organisations i.e. Federation of Master Builders, Federation of Master Craftsman, etc.

Will they agree to stage payments with the possibility of a retention of monies?

How long will the work take and will the builder agree to a reduction in the final payment if they exceed the agreed construction time (e.g. a penalty clause)?

Will the builder call in Building Control to see the relevant constructional items, i.e. excavations, drains, damp proof course, etc?

Once all this information has been gathered, you can then make your final choice – this is an important decision, so don’t be rushed into it.

Be sure the builder knows exactly what is being priced in order to avoid later conflict.

 A home being constructed








Section 6 - Work in progress

A building control officer will visit the site on the start date, meet the builder, establish if there are any apparent problems and confirm the various stages to be inspected. The officer will usually be responsible for the site through to completion. It will then be for the builder to ensure the officer is notified at the relevant stages of work.

All subsequent inspections can be made by phone, (only the start inspection needs to be in writing). If the call is made before 10am an inspection will be made on the same day - Monday and Friday.

If all statutory inspections are not requested there is a possibility that legal action may be taken and a final certificate will be withheld. This may affect the loan payment and future sale of the property.

It is in your interest to ensure all inspections are carried out. Contact the Building Control team regularly to ensure this is being done. Below is a full list of items to be inspected: (NB statutory inspections are shown with an “S”)

  • At the start of work (S)
  • When the foundation trenches are excavated (S)
  • When foundation concrete has been placed (S)
  • When the damp proof course is laid and before it is covered over (S)
  • After the site preparation hardcore has been laid (S)
  • Before the damp proof membrane in the floor has been covered (S)
  • When drains are laid but before they are covered over (S)
  • Before steel beams and their end bearings are covered up.
  • Before covering any timber structural member such as floor joists, roof carcass, timber beams, lintels, trimmers and load bearing or external studding.
  • Before covering up any concrete or steel lintels.
  • Any reinforcement prior to concreting.
  • The soakaway excavation prior to being filled with hard core (S)
  • When any problem occurs.
  • When the drains and soil and waste appliances are put on a test to show water tightness (S)
  • When the works are complete (S)


You pay for statutory inspections to be carried out and in most instances the officer may be the only professional person supervising the work.

Section 7 - Completion of work

To ensure work has been completed satisfactorily, insist the builder obtains written confirmation from the building control officer.

You may decide not to release the final stage payment until this final confirmation has been obtained.

If you do not ensure the final inspection has been carried out, it may show up on a land charge search as incomplete works and this could delay the sale of your property in the future.

It is in your interest to make full use of Building Control to help you complete the work satisfactorily.

If you would like to suggest any improvements to this information, please contact us


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