considerations report

April 2021 - Ground Extension

We highlight areas for consideration Builders, Architects Planners, Plumbers, Electricians may not highlight. We are on your side.

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Disclaimer: All the information on this page including the considerations report below does not constitute legal or financial advice, but are “Considerations”. The amounts below are estimates and is aimed at showing you the significant costs other than the Builders cost when considering a ground floor extension in London, UK.


  • This Considerations report does not take into account conservation areas.
  • These are based on London, UK Prices. Prices outside London may vary and are likely to be up to 20% – %30 lower.
Action: As you consider each line item, tick through them and add your cost in the cost box.

A. Builder Considerations

Based on the size of your Extension
  1. Builder Search: Planning Applications in your street/area: Search for recent approved planning applications and you will notice the name of the architect on the planning documents. Contact them and ask them for the builder.
  2. Checkatrade: Use to check for builders in your area.
  3. Referrals: Ask friends and family for referrals.
  4. Your Area: Always best to source a local builder.
  5. Quotes: Get 5 to 10 quotes for comparison.
  6. Certification: Ask to see proof the gas engineer and electrician are certified.
  7. Builders Company Insurance: Ask to see proof of the Builders company public liability Insurance.
  8. Contract: Ensure the contract is in the name of the company on the building insurance.
  9. Companies House: Check to see if your builder is listed as one of the directors of the builders company on companies house.

B. Archiect Considerations

Based on the size of your Extension
  1. Architect and Structural Engineer Search: Search for recent approved planning applications and you will notice the name of the architect on the documents. Contact them.
  2. Google Search: Search for Architects and Structural Engineers in your area using google maps
  3. Checkatrade: Use to check for architects in your area.
  4. Referrals: Ask friends and family for referrals.
  5. Quotes: Get 3 quotes for comparison.
  6. Certification: Ask to see proof the architect and structural engineer’s certification.

C. Building Regulations

Based on the size of your Extension
  1. Building Regulations: This is different from planning permission and a certificate is needed for extensions/conversions.
  2. Adhere to Building Regulations: You need to adhere (not necessarily get a certificate) to building regulations standards at all times, for anything done in your home.
  3. Building Regulations Certificate:
  4. You need a certificate for an extension/conversion to a property. You need to instruct either the councils building regulations department or a private building regulations company. This is your responsibility. You have to make it clear to the builder, Architect or Project manager if you expect them to to it.
  5. Commencement: You need to instruct a building regulations company or the council before the work starts.
  6. Choice: It may be best to instruct a private company than the council, as private companies are more flexible for appointments and give advice in solving issues. They are slightly more expensive.
  7. Londonbuildingcontrol: is a private company you could reach out to for building regulations.
  8. Minimum Work Carried out:  The building control team carries out the minimum amount of work needed to pass building control. They may not be able to carry out every detail as they are not on site 247. Hence, it is always best to ensure you have a trusted builder or you instruct a separate builder/Project Manager to inspect the builders work at certain intervals to have another pair of eyes on what your builder is doing.

D. Party wall Agreements

Based on the size of your Extension & if you instruct the local Council or a private company
  1. Do I need a party wall agreement: Ask your architect. If you have a semi, terraced, detached house within 1m of the boundary of your neighbour then your are likely to need a PartyWall Agreement.
  2. Timing: Do not underestimate the time it takes to go through the PartyWall Agreement with your neighbours. It can take weeks or months and delay the start of your build.
  3. Relationship with Neighbours:
  4. If you do not have a good relationship with your neighbours they can often frustrate this process. They cannot stop you from building your extension, but they could make the process more painful than it should be.
  5. Commencement: You need to have a Party Wall agreement in place before the job starts or have notified the neighbours of a commencement date and start after that date if no concerns are raised by them.
  6. Cost: You have to pay for a surveyor if the nrighbour wants a third party to arrange the partywall agreement, but the neighbour cannot force a specific partywall consultant on you. Both parties have to be in agreement.

E. Waste Location / Structure

Based on the Waste location and alteration complications
  1. Waste Location: If your main waste drain is located where you need to perform ground works such as a foundation, you need to contact Thames Water to before you commence the works and have a build over agreement.
  2. Cost: There is an additional cost with getting Thames Water involved in assessing the impact on the drain + this could increase the builders quote as there is more ground work.
  3. Partywall Agreement: 
  4. This may be included in the Party Wall agreement if you have a shared drain. That said, drains should be a concern of Thames Water.

F. Trees

Based on the Waste location and alteration complications
  1. Trees Location: If you have large trees at the rear of your garden, the structural engineer may require a deeper foundation. You may get the same request from the building control consultant.
  2. Cutting Trees: You may need a tree surgeon if you would like to cut Trees at the back of your house. Ther could be an impact on the soil if you cut a very big tree down. The soil will retain water as the tree is no longer avaiable to take up the water. If there is too much water in the soil it could impact a a foundation with a poor structure.
  3. Protected:
  4. Tall or big trees may be protected. You may have to speak to the local council if you want to cut down a large tree.

G. Under floor Heating

Based on the size of your extension
  1. Wet Under floor Heating: These a pipes that are hot water pipes from your boiler under your flooring controlled by a manifold and thermostat.
  2. Electric Under floor Heating: This is a series of electric under floor mats installed sequentially under your flooring.
  3. Wet Under floor Heating Installation Summary: 100cm of concrete, then 100cm of insulation, lay pipes, 100cm of screen then your flooring (tiles/wood etc). So you need about 30cm – 40cm below floor level to install an effective under floor unit. Some builders may try and introduce other cost effective ways to install this. Best to speak to an Under floor supplier to corroborate the suggestions by builders. Builders may want to cut corners. What they may do is install no insulation and put the pipes straight on the floor with the tiles on top, hence no insulation. The heat ends up going down not up and your tiles and property tend to be cold not warm.
  4. Thermostat and Control Unit: If you are using a smart thermostat like Nest or Hive, there are some control units that do not work with these systems. Ask your under floor heating supplier
  5. Pipe runs: Ask your Under floor supplier to provide a 2d drawing of your under floor heating run. Some builders may want to run the under floor heating on one pipe circuit on the manifold. You should run a max of 10msq per pipe run, if you do more than that, you may put too much pressure on your boilers pump and the integrity of the whole under floor unit
  6. Heating Engineer: You may need to get a separate heating engineer to install a smart thermostat. The plumber and electrician may not be able to do this.

H. Other Considerations

Based on your requirements for additions to your extension
  1. Kitchens: You may want to install a kitchen. You could spend between 8k to 30k for a new kitchen. You need to understand what you are paying for not the look in a fancy magazine or showroom. Check and to get an idea of what you are getting. This is the most expensive part of your home per sqft. People do not realise that the worktop is what really drives the look of your kitchen and the doors.
  2. Roof Construction: If you have a flat roof it may be best to get a separate company to install and guarantee your roof. Builders do not usually guarantee leakage from roofs.
  3. Insulation in walls: Some builders may cut corners and not install adequate insulation in the walls. Ask your project manager or building control office to check this on inspection.
  4. Windows:  Use: to get an idea of the cost of your window. UPVC are the standard for windows in a home. Installation cost is around £125 per window and about £175 for a small bifold (1.2m). It can be as high as £350 for a large bifold(4m) door with £125 for the initial survey. It is best not to use your builder to install windows unless they can provide a FENSA certificate and have a 10 year backed industry guarantee. Windows are a delicate part and you may want to consider using a specialist to supply and install.
  5. Bifold Doors: Use: to get an idea of the cost of your bifold door. With Bifold doors it is very simple, you get what you pay for. If you go for a cheap door, you will see the result in a year or two. The doors will likely drop, unable to close properly, draft coming in, rubber seals coming off e.t.c. Industry standard is Aluminium not Upvc for Bifold doors
  6. Internal Fittings: Have a look at our internal calculator but you need to budget internal fittings. E.g GU10 or concealed downlights, Sockets, coving, internal flooring (tiles, wood or carpet) skirting boards. Depending on your taste, this could be basic of expensive

I. Other / Notes