Choosing an Extension Builder in Sussex: From Planning to the Final Handover

If you are choosing an extension builder in Sussex, make sure every builder is pricing the same design, structural information and finish level before you compare the totals.

An extension quote can look precise while still containing major assumptions about foundations, drainage, steelwork, glazing, finishes and approvals.

Those assumptions are where budgets drift.

Do not seek final quotes from an unfinished brief

You can speak to builders early for feasibility and rough budgeting.

But a final competitive quote works best when the project information is mature enough.

Ideally provide:

• architectural drawings;
• structural drawings/calculations where available;
• planning decision or permitted-development position;
• Building Regulations route;
• specification;
• window/door schedule;
• kitchen/bathroom responsibility;
• heating/electrical scope;
• finishes;
• access details.

If the design changes after quotes are received, expect the prices to change too.

Planning permission and Building Regulations are separate

GOV.UK states that major building changes such as extensions may require planning permission, while Building Regulations deal with construction standards.

Some extensions can be permitted development.

Some cannot.

Do not rely on a builder saying “you should be fine” as your planning strategy.

Know the position before construction starts, and clarify who is responsible for any conditions attached to approval.

Check party-wall implications early

The Party Wall etc. Act can apply to work on or near shared boundaries, existing party walls and certain excavations close to neighbouring buildings.

This is separate from planning and Building Regulations.

Extension foundations are a common reason the issue appears.

If the Act may apply, deal with the process early enough that it does not become a last-minute start-date problem.

A builder can tell you about practical experience, but formal advice may need a party-wall surveyor.

Groundworks can change the entire quote

Ask every extension builder what they have assumed for:

• foundation depth/type;
• soil conditions;
• spoil removal;
• access for excavation;
• concrete delivery;
• drainage runs;
• manholes;
• retaining work;
• unforeseen obstructions.

If the final foundation design depends on site conditions or building-control inspection, understand how extra depth or different construction will be costed.

A quote with a low groundwork allowance may look attractive until excavation starts.

Structural openings need clarity

Many extensions involve removing an existing external wall and supporting the structure above.

Confirm:

• who supplies structural design;
• whether steelwork is included;
• padstones/bearings;
• temporary support;
• lifting/installing steel;
• fire protection;
• making good;
• inspection responsibility.

Do not let a major structural element sit behind a single line reading “RSJ included”.

Define the weather-tight milestone

A useful project stage is when the new structure is genuinely weather-tight.

But define it.

Does it mean:

• roof complete;
• windows/doors installed;
• external openings sealed;
• temporary weathering only?

This matters if payments are linked to milestones.

The clearer the milestone, the less room for disagreement.

Decide who is supplying high-value items

Extensions often contain expensive client-choice items.

Examples:

• bifold/sliding doors;
• rooflights;
• kitchen;
• sanitaryware;
• flooring;
• lighting;
• appliances.

If you supply them, ask:

• who measures;
• who checks suitability;
• who stores them;
• what happens if delivery is late;
• who deals with defects;
• what installation labour is included.

Client-supplied items can save money or create programme risk depending on how they are managed.

Compare the programme, not just the start date

A builder saying “we can start next month” sounds attractive.

Ask:

• approximate duration;
• key stages;
• expected periods without kitchen/heating;
• long-lead items;
• holiday periods;
• weather-sensitive stages;
• subcontractor sequencing;
• what could delay the job.

If you remain in the house, ask when the project becomes most disruptive.

Variations need discipline

Extensions are particularly vulnerable to scope creep.

You may decide to:

• move sockets;
• upgrade doors;
• add lighting;
• change flooring;
• alter the kitchen;
• open another wall.

None is necessarily a problem.

The problem is making dozens of changes without tracking cost and time.

Agree a variation process before the first change.

Landlords: decide whether the property should be occupied

For investment property, sometimes vacant works are far easier.

Consider:

• service interruptions;
• dust/noise;
• security;
• access;
• insurance;
• tenant rights/communications;
• temporary kitchens/bathrooms;
• programme uncertainty.

Do not assume an extension around an occupied tenancy will operate like a minor repair visit.

Completion is more than the last day on site

Before final payment, agree the handover items.

Potentially:

• snagging;
• Building Regulations completion evidence;
• electrical certificates;
• gas/heating documents;
• warranties;
• product manuals;
• keys;
• drainage information;
• photographs of concealed services if available.

GOV.UK notes that completion certificates can matter later when selling a property.

Store the project file somewhere permanent.

Check insurance and responsibility for the site

For a major extension, ask the builder what insurance they carry and whether any special cover is needed because the property will remain occupied or because structural work is substantial.

Also tell your own home insurer before work starts if your policy requires notification for major building works.

The important point is to avoid discovering after an incident that the homeowner, builder and insurer all assumed someone else carried the relevant risk.

Think about neighbour and access logistics before the start date

Extensions often affect more than the footprint of the new room.

Consider:

• scaffold near boundaries;
• skips and deliveries;
• shared drives;
• temporary fencing;
• concrete pumps;
• crane or steel deliveries;
• parking;
• noise;
• protection of neighbouring surfaces.

If access depends on a neighbour, do not leave the conversation until the morning a delivery arrives.

Practical cooperation does not replace formal Party Wall requirements where they apply, but it can prevent avoidable friction around day-to-day construction.

Check insurance and responsibility for the site

For a major extension, ask the builder what insurance they carry and whether any special cover is needed because the property will remain occupied or because structural work is substantial.

Also tell your own home insurer before work starts if your policy requires notification for major building works.

The important point is to avoid discovering after an incident that the homeowner, builder and insurer all assumed someone else carried the relevant risk.

Think about neighbour and access logistics before the start date

Extensions often affect more than the footprint of the new room.

Consider:

• scaffold near boundaries;
• skips and deliveries;
• shared drives;
• temporary fencing;
• concrete pumps;
• crane or steel deliveries;
• parking;
• noise;
• protection of neighbouring surfaces.

If access depends on a neighbour, do not leave the conversation until the morning a delivery arrives.

Practical cooperation does not replace formal Party Wall requirements where they apply, but it can prevent avoidable friction around day-to-day construction.

Using TradeKind

You can browse extension builders across Sussex and see how businesses describe their services and coverage.

Or post your extension project.

A strong post says what stage you are at:

“Single-storey rear extension in Lewes. Planning approved, Building Regulations drawings and structural calculations available. Looking for a contractor to deliver shell through to plastered finish; kitchen supplied separately.”

That lets builders quickly decide whether they want that kind of project.

Final decision test

Before choosing the contractor, check:

• design is defined;
• approvals route is understood;
• party-wall position is considered;
• groundwork assumptions are explicit;
• structural work is detailed;
• high-value supply responsibilities are clear;
• programme and milestones make sense;
• payment stages follow progress;
• variations have a process;
• handover/certificates are understood.

An extension is too large a project to manage by memory and WhatsApp alone. The clearer it is before the first excavation, the better.