The Essential Guide to Building an Extension: Your Complete Extension Planner

Why You Need an Extension Planner

Whether you're choosing to project manage your extension yourself or seeking to understand the various stages of building an extension, having an extension planner is crucial. It covers all the different jobs that need to take place and where they fit into the overall scheme. This knowledge will give you a better grasp of what's ahead and the resources you'll need for each step.

While the extension planner for each project will be tailored to suit your specific needs, the key stages will follow a logical program of works. This ensures that each element is completed on time and within budget. There are 12 basic steps to anticipate what needs to happen and when.

Any construction program should clearly outline a project’s scope of work, identify timescales, lead times, development phases, and durations. Project budgets and cash flow can also be incorporated to assist with financial control.

For larger projects, such as a double-storey extension, the main change will be the timings. The work required will still follow the same logical steps. Using an extension planner is also vital if you will be living in the property while works are in progress. This way, you can anticipate when to expect the most disruption.

Another significant benefit of having an extension planner in advance is that it helps you establish a schedule of works. This schedule can be sent out to tender to get a fair set of quotes. It is essential that a schedule of works and specification document is submitted to all builders who are tendering for the work. If you only provide a set of drawings without a specification document, each builder will assume their own specifications for the materials. These assumptions can lead to significant cost variations.

The schedule of works should list all elements to be priced. For example, the installation of finished flooring may not be shown on the drawings, but you may want this included in the cost breakdown. The schedule of works and the specification document will eliminate any assumptions and provide a true comparison.

Step One: Finalise Conditions and Arrange Insurance

Before starting to build, ensure you have given appropriate consideration to the level of pre-construction information required for the scope and scale of your project. Make sure you have discharged any ‘pre-commencement’ planning conditions before work begins on-site. Planning conditions can relate to service connections, trees, or access, and must be resolved before building begins. Otherwise, you could face enforcement action.

Also, check that your existing home insurance policy covers any works undertaken as part of an extension project. Avoid any nasty surprises later by contacting your insurance provider to clarify what is and isn’t covered. To ensure your home is properly insured during substantial home improvements, you’ll often need a specialist policy, such as those offered by Self Build Zone.

Step Two: Stabilise Building and Make Safe

For those adding an extension while renovating a house, the existing building must be made safe before any work commences. It will become a ‘place of work’ if you employ a builder, tradespeople, or have anyone else helping you on-site. This means cutting off the power and water to any building services to be removed and providing temporary power and water supplies for the construction works as part of the ‘site setup.’

An asbestos survey is essential to ensure the building is free of potentially life-threatening contamination. Any asbestos found must be removed safely, following guidance from the Health and Safety Executive (HSE).

Step Three: Strip Back and Salvage

Once the site is safe, demolition work can begin to dismantle parts of the building that are being removed or altered. Used building materials often have value. Salvaging them for reuse on your site or selling them through a salvage yard will help with the sustainability of the project.

When removing old extensions to add a new one, instead of filling skips with rubble, set aside an area to stockpile materials. You may need to hire a specialist machine to grind rubble down into correctly sized material that can be used in the extension foundations. This can save money on materials and skip hire, demonstrating a commitment to sustainability.

When replacing an old extension, a ‘soft strip out’ contract is often the first stage of a more extensive renovation or extension project. This can involve removing all old building services (wiring, heating, and plumbing), stripping old decorative finishes back to plasterwork, or removing plaster back to the brick or masonry to facilitate repairs and rebuilding work.

This stage usually involves ensuring you've included scaffolding costs to provide access. If a full scaffold is required to cover all or part of the building due to roof removal, a structural engineer’s design is legally required. Be sure to notify the local authority of the demolition works as well.

Step Four: Treat Any Structural Issues

Before or alongside building work to alter or extend your home, it is crucial to solve any issues such as rising damp, insect infestation, and fungal infestation. Remove all timber elements affected by dry rot and treat the building with appropriate fungicide. Unlike the less dangerous wet rot, dry rot can spread through damp brickwork and plaster, making it one of the most challenging types of rot to treat.

Although damp treatments, insecticides, and fungicides are available on the DIY market, it is best to leave this work to specialist contractors who will provide an insurance-backed guarantee. If you find dry rot, you may need to extend the time allocated for building your extension.

Step Five: Major Structural Works

This is the main part of the building contract and involves heavy building work to add your extension or remodel an existing one. If you are adding a new extension and not removing any previous buildings, this is when your main contractor or builder will set up the site and begin bringing in the necessary tools, machinery, equipment, and materials.

On larger projects, facilities such as a site cabin and a toilet will be set up in your garden, although smaller extension ideas may not require as much infrastructure. Along with the logistical setup, it’s also when your builder will mark out the extension on the ground.

While you shouldn’t need to get your tape measure out and check, it’s worth ensuring it aligns with your expectations from the plans. If you have any concerns, make sure to voice them. Waiting to speak could lead to costly changes later or even result in building control or planning not signing off the work.

Once you're ready to go, this is where the work truly begins. Groundworks, foundations, walls, and various types of extension roof construction will take place to form the completed superstructure for your extension. Door and window openings will be created, and it's essential to ensure they are all in the correct locations.

It pays to have either you, your architect, or project manager monitoring what's happening on-site to reduce unexpected problems. Initially, groundworkers will be on-site, quickly followed by bricklayers, carpenters, roofers, and general building operatives to install drainage systems, service risers, and insulation. Note that works to connect new drainage runs to the existing drainage network will usually be left until the heavy building work is complete and the scaffold has been dropped.

Once the roof covering is completed and any external cladding is in place, the gutters and downpipes can be fitted. Any external decoration and mastic sealing work needed outdoors will also be undertaken. The scaffold can then be dropped unless you're adding a double-storey extension, in which case you’ll need more scaffolding as the build progresses.

Throughout this stage, expect several visits from building control to check that the works in progress align with what’s been agreed. You have the option to work with a private approved building inspector instead of the local authority. The plans will need to be submitted to them at least five days before starting on-site, so it’s best to ask your builder who they prefer working with. Most will work directly with the local council Building Inspector.

Step Six: Make the Shell Weathertight

Before internal works commence, the building should be made secure and weathertight once the superstructure is in place. This means the roof is completed (at least covered if not fully tiled) and all window and door openings are closed up. In most cases, this will mean the roof is tiled, and the doors and windows are fully glazed. If these are not yet on-site, or the glazing is considered too vulnerable to install while works are still underway, temporary works can be used to close up the building.

To accelerate the building program, it is possible to make parts of a building weathertight, often the top attic storey or upstairs storey, and seal it off to commence first fix while the heavy building work continues on lower storeys. Often, home improvement projects such as extensions and loft conversions are made weathertight and then completed to plastered-out stage before the demolition work to ‘knock through’ to the existing dwelling is undertaken. This can be an excellent way to separate building works from the existing home, especially if you are still living on-site during the project. During the knock-through stage, it is best to move out for a few days or one to two weeks.

Door linings and window boards, followed by windows and doors, will need to be added next to finish securing the exterior of your extension. It’s also the point at which any outstanding work to join your extension to your existing home may take place. If undertaking a larger project involving external walls being removed, this will have happened sooner.

Step Seven: First Fix

If you’ve left it until now to decide on your kitchen design, bathrooms, lighting, heating, and electrical plans, you’re behind schedule. These ideally need to be decided when your plans are being drawn up. No one wants to channel out a freshly plastered wall in a few weeks when they realize they forgot to add wall lights or a socket for the oven.

Depending on how a new extension impacts your existing home’s wiring and plumbing, this will also be the stage when any changes are made to the rest of your home so that the two spaces start to work together. Before commencing first fix, all stud partition walls (timber or metal studwork that will be finished with plasterboard later) also need to be completed. An external power supply for outdoor lighting, electric gates, water features, etc., also needs to be in place at the first fix stage.

Why? First fix primarily involves all internal works that need to be completed before screeding the floors and plastering the walls. Insulation and building services (wiring, plumbing, heating pipework, and control wiring, mechanical ventilation) will all be concealed by the screed and plaster finishes. Therefore, it is essential to have finalized all building service layouts before starting the first fix stage.

First fix also includes fitting the door and window linings around openings so that plasterers have a profile to work to. Once complete, the floors will be screeded to the levels ready for the floor finishes. Before screeding commences, it is essential to have chosen all the floor finishes. Different flooring finishes such as flagstones, tiles, and carpet will all require different depths, and it's a flooring mistake that can be tricky, costly, and messy to rectify.

Step Eight: Plastering and Making Good

This stage begins to transform a building site into something resembling a home. Before the plasterers commence work, each room will be prepped. The beads (stop bead, angle bead, shadow gap bead) will be fitted, and wire mesh and scrim will be used to close up any gaps. Any existing plasterwork that requires repair or a new skim coat of finishing plaster will also be prepared.

The ceilings are battened out, and sheets of plasterboard are screwed into position (known as ‘tacking’ work). Shower enclosures and wet areas are finished with a waterproof tile backing board. The plasterers will then work through the home room by room, starting with the ceilings and moving into the walls. They will leave behind beautifully smooth polished surfaces unless you specify an alternative finish such as stucco plastering, with sharp angles around openings and external corners. Any decorative plasterwork, such as coving, cornice, and ceiling roses, can then be applied.

Whether or not the stairs are installed at this stage is a difficult decision. Plastering to the strings can give a better finish, but it is challenging to protect the stairs from damage at this stage. If an expensive staircase idea has been selected, it is best to leave installation to the second fix stage unless it can be fully protected from water, paint spills, and heavy traffic.

Step Nine: Lay Fixed Flooring and Tiles

Before the second fix (installing skirting and architrave, hanging doors, and fitting sanitaryware), all fixed flooring needs to be laid. This includes tiled and most types of wood flooring, but not soft floor coverings like carpet or lino. All floor finishes need to be protected from damage by foot traffic or covered in paint from the decorating stage. Wooden flooring should not be laid until the screed substrate has dried out fully and the moisture level reading is in line with the manufacturer’s guidance.

Step Ten: Second Fix

Prior to this stage, it is common practice to prime all the fresh plasterwork as soon as it has dried out with a mist coat (watered-down emulsion). This ensures adhesion of the paint finishes and is quicker to apply with a roller without worrying about other finished surfaces.

Once the walls are prepped, jobs such as hanging internal doors, fitting ironmongery (handles and knobs), fixing architrave and skirting, window boards/cills, fitting timber paneling or other decorative details, and fitting a kitchen will transform a blank canvas into a livable space. Fireplaces and wood-burning stoves can be installed, and sanitaryware and brassware (taps and showers) will make your bathrooms usable. Faceplates for sockets and various types of light switches will appear on walls, along with your ceiling and wall lights. If you've run cabling for a home security system, this is when it will be commissioned.

Step Eleven: Decoration

Once the fixtures and fittings are in place, it’s time to put your stamp on your home. To achieve a high-quality finish, remember that the work is 80% preparation and only 20% painting. Plaster finishes need to be rubbed down and filled; timber finishes need to be rubbed down, knotted, filled, and primed; MDF surfaces need to be primed. Paintwork should be built up in 2-3 coats and carefully cut in. Sloping ceilings are best painted in the same color as the walls to avoid difficulties in cutting in a straight line.

This stage also involves applying mastic sealant to bathrooms, kitchen worktops, and hearths, as well as filling any other gaps or areas that might require waterproofing. External decoration should have been completed while the scaffold was still up, but at this stage, it is normal to complete outstanding external mastic work to fill around doors and windows, expansion joints, etc. Once your walls have received a mist coat, the real decorating can start.

Step Twelve: Snagging and Practical Completion

Once work is finished, it is crucial to write up an extension snagging list. Any leaks, problems with electrics, and sticking doors and windows need to be reported to the relevant trade as soon as possible after finishing. If you have used a main contractor, there will be a defects liability clause in the contract and a payment retention. This ensures that the builder returns to complete defects work, usually six months following practical completion. Once the works have been completed, the remaining balance of retention monies can then be released.

If you have managed the project yourself, the cost of correcting any defects will be your responsibility unless you have agreed on a retention with individual subcontractors. All building work undertaken is still, however, covered by consumer law. Practical completion can run concurrently with snagging, depending on the severity of any defects. It is typically defined as the point when the building work is complete, save for any minor defects. If you agree that the work is completed as agreed (excluding minor snags), you can take possession back from the builder and officially move in. Expect to receive all manufacturers’ and installers’ guarantees, commissioning certificates, and the Electrical Installation Certificate.

Whoever is dealing with building control will also need to provide all required information to obtain a building regulations completion certificate, including the ‘as built’ Energy Performance Certificate (EPC). Once snags are completed, if there is a retention clause in your building contract, 50% of the retention monies held against defects should be released within the agreed timeframe.

All that's left to do now is to sit back, relax, and enjoy your new extension.

Final Tips for Your Extension Schedule

One final tip when it comes to your extension schedule? Don’t forget to account for any work needed to landscape the garden, especially if you are looking for ways to make your extension work with your garden. Once any scaffolding has been dropped, try to ensure any structural landscaping work needed to form the contours of the garden, driveway design, and paths is completed while heavy machinery is still on-site. Although this is not always practical, especially on a tight site, it can be more cost-effective and less disruptive than trying to get the work done once the extension is finished.

Jobs to consider, in addition to garden paths and driveway ideas, include screening lawns and beds to remove stones, leveling any lawns, spreading topsoil, and even starting your planting scheme. Equally important is your garden lighting ideas, particularly if you are rewiring your home as part of your extension plans and need to include any new garden lighting in the cabling and overall electrical load.

If you found this article useful, please check out the other articles on my blog. If you need any assistance with a planning application, etc., please get in touch.