Home Extension Planning Preparation Checklist

Write down what you know about how the house was built and what has already been altered or added, including anything a previous owner did before you moved in.

Earlier alterations usually explain odd floor levels, blocked openings and patched brickwork, and a designer will ask about them before proposing anything.

Note where your title documents show the boundary running, and where fences, hedges or walls actually sit on the ground compared with that recorded line.

A boundary that has drifted from the recorded line changes what can sensibly be drawn, and it is far easier to establish now than after a layout is agreed.

Boundary interpretation is a legal matter; raise it with your conveyancer or a qualified professional rather than relying on where the fence happens to stand.

Sketch roughly how the existing roof drains at the rear and sides, noting where gutters run, where downpipes land and whether any of them discharge towards a neighbouring plot.

An extension almost always interrupts an existing rainwater route, and having that visible on paper early prevents a late redesign of the roof edge.

Record every step or change of floor level between the rooms nearest the proposed extension, including the step out to the garden if there is one.

Level changes drive the whole ground build-up, and owners often assume a flat run through the enlarged plan when the existing house does not offer one.

Note where the incoming water, electricity and gas supplies and their meter boxes sit, and whether any of them are on a wall a new structure might enclose.

Supply positions and meter boxes on an affected wall raise supplier questions that follow their own process, entirely separate from the design conversation.

Only the relevant utility supplier can say what moving or enclosing a meter involves; ask them directly and keep a written note of the answer.

List any existing signs of damp, cracking or movement on the walls near where you are thinking of building, and photograph each one before anything changes.

Existing defects shape what a professional advises and how they judge the risk of the unknown, and photographs taken beforehand are far more useful than a recollection.

List the activities the new space has to hold on an ordinary evening and on the busiest occasion you can imagine, then note which of the two you would rather it suited.

Spaces designed only for the rare gathering often feel wrong the rest of the time, and saying which case wins gives a designer something to work against.

Decide and record whether the new space replaces something you already have or adds to it, because that determines what then happens to the room it replaces.

Owners frequently gain a large new room and quietly lose a dining room or a study without ever having decided to, which shows up only once the plan is drawn.

Write down whether the new space has to work with a door closed on it, for noise, cooking smells or heat, or whether it is always open to the rest of the plan.

Open plan and closed plan lead to different structural and heating conversations, and this is one of the few briefing decisions an owner can settle alone.

Note the largest pieces of furniture that must fit, measuring them roughly yourself, and mark which are fixed choices and which you would replace.

A table or sofa that cannot be given up is a real constraint, and stating it early is more honest than discovering it once the walls are set.

Consider where the storage that currently lives in the area being built on would go once the extension takes its place, and write down what it holds.

Garden stores, under-stair cupboards and side passages absorb a great deal, and their contents rarely vanish just because the space does.

Write down who uses the new space when nobody else is in the house, and whether that changes how it should connect to the front door and the stairs.

A room used by one person late at night makes different demands on circulation and security than one only used when the household is together.

Identify which existing external wall would become an internal junction, and write down every window and door in it that would be lost, blocked or changed.

Owners tend to picture the new room and forget that the old wall carries openings, radiators, sockets and a rainwater pipe that all have to go somewhere.

Note which existing rooms would end up with no outside wall at all, and ask how daylight and ventilation would then reach them.

The middle of an extended house is where light disappears, and identifying the affected rooms early gives a designer a problem to solve rather than an afterthought.

How an enclosed room is lit and ventilated is a matter for a qualified professional and the relevant local requirements; raise it as a question rather than assuming an answer.

Walk the route from the front door to where the new space would be and write down how many doors and turns it involves, and whether it passes through a room someone might want to close off.

Circulation that only works when every internal door is open is a common outcome of extending, and it is easiest to notice by walking it.

Record which way each door near the junction swings, and whether that swing would block a radiator, a light switch or the only sensible place for a tall unit.

Door swings are easy to change on paper and awkward to change once joinery and heating positions have been fixed around them.

Note the ceiling height of the existing rooms at the junction and decide whether you would accept a visible change in height where old meets new.

Matching an existing ceiling line is not always possible, and knowing in advance whether you mind saves an argument at the drawing stage.

Ask what happens to the heating in the retained rooms once an external wall becomes an internal one, and where the radiators currently on that wall would move to.

Radiators on the wall that disappears have to be relocated, and their new positions interact with furniture layouts you may already have in mind.

Heat output and pipe routing are for a qualified heating professional to work out; treat this as a topic to raise, not a conclusion to reach from a checklist.

Photograph every manhole and access cover in the garden, side passage and driveway, and note roughly where each sits relative to the house wall.

Covers mark drainage runs that a designer needs to know about, and photographing them now avoids a search once the ground is disturbed.

Note where rainwater from the existing roof goes once it reaches the ground, and whether you can see a gully, a channel or nothing obvious at all.

Extending changes both the roof area and the amount of hard surface, so the existing arrangement is the starting point for that conversation.

Write down the position and rough spread of any tree, large shrub or thick hedge within the area you are considering, including specimens in neighbouring gardens.

Nearby trees influence both what can be designed and what a professional will want investigated, and neighbouring trees are frequently forgotten.

Trees can carry protection and their effect on ground conditions is for qualified professionals and the local authority to assess; confirm status locally before assuming anything.

Record which way the ground falls across the proposed footprint, using a rough eye-level check from the house wall out to the far end of the area.

A slope the household stopped noticing long ago can change the whole approach to floor level, steps and retaining, and owners rarely mention it unprompted.

Note anything you know about earlier ground work on the plot, such as a filled pond, a demolished outbuilding, a removed tree or a previously dug patio base.

Made ground and old foundations are among the most common causes of a changed approach once digging starts, and local knowledge is genuinely useful.

Ask whether any drain crossing your plot also serves another property, because a shared run brings other parties into the conversation.

Shared runs are common in terraced and semi-detached streets and change who has to be consulted before anything is designed over them.

Ownership and status of a drain run is determined by the relevant authority or drainage undertaker; ask them directly and record what you are told.

Confirm locally what consent arrangements apply to your property and your type of proposal before commissioning any drawings, and note who you spoke to.

Requirements vary between areas and between properties, and an informal opinion from a neighbour or a builder is not a substitute for asking the authority.

Only the local authority can tell you what applies to your property; treat every other source, including this checklist, as a prompt to ask them.

Ask whether your property or its area carries any designation that changes what has to be applied for, and record exactly how that was described to you.

Designations change the whole route a proposal takes, and finding out after a design already exists is one of the more painful ways to learn it.

Check whether any earlier consent granted on the property came with conditions that still bind what can be done now, including on previous extensions.

Conditions attached to a past approval survive changes of ownership and can restrict exactly the thing you are planning.

Ask what drawings, forms and supporting information the authority expects to receive, and in what format, before anyone starts producing them.

Knowing the expected package prevents work being produced twice and helps you understand what you are asking a professional to provide.

Agree in writing who is responsible for making any application and for answering questions raised during the process, and how you will be kept informed.

Applications stall most often because nobody was clearly responsible for responding, and this is settled easily at the outset.

Record every answer you receive about consent in one place, noting the name of the person who gave it and the reference of any conversation or case.

Verbal guidance is easily misremembered, and a single written record is what makes the next professional conversation efficient.

Write down which neighbouring walls, fences, garages or outbuildings sit close to the proposed footprint, and roughly how close each of them is.

Proximity determines which conversations and processes have to happen, and it is the first thing a professional will look for on a site visit.

Note whether any part of the work would need access over land you do not own, including for scaffolding or for reaching a flank wall, and who you would need to ask.

Work that can only proceed if somebody else agrees is a dependency worth naming at the outset, while there is still time to design around a refusal.

Ask whether a wall you are thinking of building against, cutting into or excavating near is shared, and what process that triggers for a project like yours.

Work touching or near a shared wall follows its own route that runs alongside any consent process, and it is regularly discovered too late.

Procedures for work affecting a shared or nearby wall are set by the relevant local requirements; ask a qualified professional to confirm what applies before work is planned.

Note where neighbours look out from and where they sit outside, listing the windows and doors that face the proposed footprint.

Outlook and overlooking generate most neighbour objections, and knowing about them before drawings exist gives a designer room to respond.

Record any informal understanding you already have with a neighbour about fences, gutters, access or planting, and put it in writing while relations are easy.

Understandings that were never written down are the ones that come apart under the pressure of building work.

Note whether your gutter, roof edge or foundation currently oversails a neighbouring boundary, because a new structure may raise the same question again.

An existing overhang that has never been questioned can become contentious the moment something new is proposed alongside it.

Note where the boiler or heat source, the hot water cylinder and the consumer unit sit, and whether any of them stand in the area being changed.

Relocating plant is a separate piece of work with its own constraints, and its position often quietly dictates the layout of the new space.

Ask whether the existing heating, hot water and electrical arrangement can serve the enlarged house, and treat the answer as a professional judgement rather than an assumption.

Adding rooms adds demand, and the answer determines whether a separate strand of work has to run alongside the building work.

Capacity of a heating or electrical installation must be assessed by a suitably qualified professional; this checklist can only tell you to ask the question.

Write down whether the new space is intended to contain water, such as a kitchen, a utility area or a cloakroom, because that decides whether drainage has to reach it.

Getting waste away from a new wet area is often the single biggest constraint on where it can go, and it is settled by the brief rather than by taste.

Note the external cables, boxes and brackets fixed to the wall that would be built against, including aerial, broadband, alarm and satellite cabling.

Each of these belongs to a different provider or trade, and unpicking them is a small task that is only irritating when it is discovered late.

Start a rough room-by-room list of where you would want sockets, switches and lighting in the new space, based on how you actually use similar rooms now.

These positions are usually decided under time pressure on site, and an owner who has thought about them beforehand gets a better result.

Ask what happens to any outside tap, external light, extract vent or flue terminal that currently sits on the wall the extension would cover.

Vents and flue terminals in particular cannot simply be built over, and they often need a decision before the footprint is finalised.

Never plan around a flue terminal or combustion vent yourself; that belongs with a suitably qualified heating professional and should be asked about early.

Gather any existing drawings of the house, including anything handed over when you bought it and anything produced for earlier alterations.

Even inaccurate old drawings save time, because they show what was intended and where earlier work was recorded.

Collect the survey or condition report you received when purchasing, together with any specialist report obtained since then.

These often record construction type, defects and alterations that you would otherwise be guessing at from memory.

Take a set of photographs of every elevation and of the rooms at the proposed junction, ideally on both a dull and a bright occasion.

Light behaves differently across a house than owners expect, and a paired set makes the daylight conversation concrete rather than theoretical.

Separate your wish list into what is fixed, what is flexible and what you would drop if something had to go, and write the three lists out separately.

Designers work far better against a stated hierarchy than against an undifferentiated list of everything the household has ever wanted.

Write down who in the household makes the final call when two of you disagree, and how that decision gets recorded so it is not revisited weekly.

Unresolved internal disagreement is one of the most common causes of drift and rework once a design is under way.

Note the questions you already know you cannot answer yourself, and keep them as the agenda for the first professional conversation.

Arriving with a written list of unknowns turns an introductory meeting into a working one and makes the advice you receive more targeted.