Drainage and Sewer Issues

Drainage is one of the most common areas where unexpected costs arise.
 

You may need to allow for:
 

• relocating drains;

• replacing old pipework;

• moving manholes;

• new inspection chambers;

• a CCTV drainage survey;

• repairing damaged drainage;

• upgrading rainwater disposal;

• additional soakaways;

• and water-authority approval where relevant.
   

A drain running beneath the proposed extension may affect the foundation design or require additional work.
 

Finding out as much as possible before the build begins can reduce uncertainty.

 

Foundations and Ground Conditions

The cost of foundations is not always predictable from the outset.
 

Additional work may be required where:
 

• the ground is poor;

• trees are close to the extension;

• clay soil is present;

• existing foundations are shallow;

• drainage trenches affect the new structure;

• ground levels vary;

• or unexpected obstructions are discovered.
   

Possible extras may include:
 

• deeper foundations;

• additional excavation;

• concrete;

• reinforcement;

• specialist foundation solutions;

• underpinning;

• and additional structural advice.
   

Your builder may include an allowance based on reasonable assumptions, but the final cost can change once excavation begins.

 

Trees, Roots and Landscaping

Trees can affect both planning and construction.
 

You may need to consider:
 

• tree surveys;

• root-protection measures;

• deeper foundations;

• removal of unsuitable planting;

• temporary protection during construction;

• replacement planting;

• and making good the garden after the works.
   

The construction process may also damage lawns, patios, paths and planting areas.
 

Allow for landscaping and garden repairs when setting the final budget.

 

Access to the Site

Access can make a major difference to construction costs.
 

A property with a side passage, driveway or easy rear access may be more straightforward to build.
 

A mid-terrace property with no side access may require materials and waste to pass through the house.
 

Additional costs may arise from:
 

• protecting floors and walls;

• carrying materials through the property;

• additional labour;

• smaller deliveries;

• restricted working hours;

• parking suspensions;

• skip permits;

• scaffold licences;

• and temporary access arrangements.
   

The builder should understand the access limitations before providing a detailed quotation.

 

Demolition and Waste Removal

Do not overlook the cost of removing what is already there.
 

This may include:
 

• an old conservatory;

• an existing extension;

• a patio;

• garden walls;

• sheds;

• old drainage;

• internal walls;

• ceilings;

• kitchen units;

• flooring;

• and redundant services.
   

The quotation should explain whether demolition, skips and waste removal are included.
 

Some sites need several skips.
 

Others may require grab lorries, smaller waste collections or more labour because access is limited.

 

Asbestos

Older homes may contain asbestos-containing materials.
 

This can be relevant where the project involves:
 

• removing an old conservatory;

• disturbing textured coatings;

• stripping older floor tiles;

• altering ceilings;

• removing soffits;

• demolishing outbuildings;

• or carrying out wider refurbishment works.
   

If asbestos is identified, specialist removal may be required.
 

This can create an additional cost and affect the programme.
 

A survey before intrusive work begins may help avoid an unpleasant surprise.

 

Temporary Protection and Making Good

Construction work can affect more than the extension area.
 

You may need to allow for:
 

• protecting existing floors;

• temporary partitions;

• dust screens;

• temporary doors;

• making good walls and ceilings;

• repainting adjoining rooms;

• repairing damaged paths;

• replacing skirting boards;

• and redecorating areas affected by structural work.
   

If the rear wall is being removed, the existing kitchen and dining area may need more extensive repair and redecoration than expected.

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Kitchens and Utility Rooms

 

The kitchen is often one of the largest additional costs.

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The builder’s quotation may include the construction work but exclude:

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• kitchen units;

• worktops;

• appliances;

• sinks and taps;

• kitchen installation;

• specialist lighting;

• fitted storage;

• utility-room units;

• and decoration.

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A modest kitchen and a high-end fitted kitchen can have very different budgets.

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Consider the kitchen design early because it may affect drainage, electrics, glazing and structural decisions.

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Bathrooms and Ground-Floor WCs

 

Many extensions include a ground-floor WC, shower room, utility room or en-suite bathroom.

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Allow for:

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• sanitaryware;

• taps;

• shower fittings;

• tiling;

• extraction;

• plumbing;

• drainage;

• heating;

• lighting;

• mirrors;

• storage;

• and installation.

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A small room can still involve a significant number of separate trades and materials.

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Glazing, Doors and Rooflights

 

Large glazed openings can make an extension feel bright and modern, but they can also be a substantial part of the budget.

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Possible costs include:

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• sliding doors;

• bifold doors;

• steel-look doors;

• aluminium windows;

• entrance doors;

• rooflights;

• roof lanterns;

• bespoke glazing;

• internal glazed partitions;

• and installation.

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Check whether these items are included in the builder’s quotation or whether you will arrange them separately.

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Also allow for lead times.

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Ordering glazing too late can delay the programme.

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Flooring

 

Flooring is sometimes excluded from early quotations.

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Remember to allow for:

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• floor preparation;

• screeding;

• levelling;

• timber flooring;

• tiles;

• luxury vinyl;

• carpet;

• underlay;

• trims;

• skirting boards;

• and installation.

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If you are using underfloor heating, the floor build-up should be coordinated properly before the works proceed.

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Heating and Underfloor Heating

 

An extension may affect the existing heating system.

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You may need to consider:

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• new radiators;

• underfloor heating;

• boiler capacity;

• heating controls;

• pipework alterations;

• relocating radiators;

• hot-water demand;

• and the position of the existing boiler.

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An older boiler may not be suitable for the enlarged property.

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If a boiler upgrade is required, this can become a significant additional cost.

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Electrical Work and Lighting

 

Electrical requirements are easy to underestimate.

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A modern kitchen-diner may need:

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• sockets;

• appliance connections;

• island power;

• lighting circuits;

• pendant lights;

• recessed lights;

• external lighting;

• garden power;

• data points;

• smoke alarms;

• extract fans;

• and charging points.

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Think about the lighting plan early.

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Moving sockets and lights after plastering can create unnecessary additional work.

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Decorating

 

Some builder quotations exclude decorating.

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Even where decorating is included, check the scope carefully.

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You may need to allow for:

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• mist coats;

• final paint finishes;

• feature walls;

• woodwork;

• internal doors;

• skirting boards;

• architraves;

• ceilings;

• existing adjoining rooms;

• and external finishes.

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An extension often affects more of the original house than homeowners initially expect.

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External Works

 

The extension may be complete internally while the garden and external areas still need attention.

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External works can include:

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• patios;

• paving;

• steps;

• retaining walls;

• drainage channels;

• external lighting;

• fences;

• gates;

• lawns;

• planting;

• repairs to paths;

• and making good areas used for access.

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If the new doors open onto the garden, the relationship between the internal floor level and the external patio should be considered carefully.

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Storage and Temporary Accommodation

 

Some homeowners remain in the property during the build.

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Others move out temporarily.

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Possible costs include:

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• temporary accommodation;

• storage;

• removals;

• additional travel;

• eating out;

• temporary kitchen arrangements;

• laundry;

• and cleaning.

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These costs may appear modest individually but can add up over several weeks or months.

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VAT

 

Check whether VAT has been included in the quotations you receive.

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A quotation that excludes VAT may look significantly cheaper than a quotation that includes it.

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When comparing prices, make sure that you are comparing like with like.

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Ask each contractor and professional adviser to state clearly whether VAT applies.

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Delays and Price Changes

 

Construction projects do not always proceed exactly to plan.

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Possible causes of delay include:

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• poor weather;

• late deliveries;

• material shortages;

• changes requested by the homeowner;

• unexpected structural issues;

• drainage problems;

• Building Control queries;

• party-wall matters;

• and specialist subcontractor availability.

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A longer programme can create additional costs, particularly if you are renting temporary accommodation or financing the works.

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Changes During Construction

 

One of the easiest ways to increase the cost of an extension is to make repeated changes after the build has started.

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Changes may affect:

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• labour;

• materials;

• structural work;

• electrical layouts;

• plumbing;

• glazing;

• kitchen design;

• finishes;

• and the construction programme.

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Try to make the important design decisions before work begins.

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Where changes are unavoidable, ask for the cost and programme impact to be explained in writing before the additional work proceeds.

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Keep a Contingency Allowance

 

Even with good drawings and a detailed quotation, it is sensible to keep a contingency allowance.

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Older homes can reveal unexpected issues once work begins.

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A contingency can help you deal with:

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• hidden drainage;

• poor ground conditions;

• structural repairs;

• damaged existing walls;

• additional steelwork;

• electrical upgrades;

• plumbing alterations;

• and changes to the specification.

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The appropriate allowance depends on the property and the complexity of the project.

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A straightforward extension to a modern home may carry fewer uncertainties than a substantial renovation of an older period property.

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Questions to Ask Before You Appoint a Builder

 

Before accepting a quotation, ask:

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• What has been included?

• What has been excluded?

• Does the price include VAT?

• Are foundations based on an assumed depth?

• Has drainage been allowed for?

• Are structural steels included?

• Are glazing and external doors included?

• Are kitchen supply and installation included?

• Are electrics and plumbing included?

• Is decorating included?

• Are skips and waste removal included?

• Are scaffolding and access costs included?

• Are provisional sums clearly identified?

• How will variations be agreed?

• What contingency should I retain?

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A clear quotation does not eliminate every risk, but it makes the budget easier to understand and manage.

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Plan the Whole Project, Not Just the Build

 

A successful extension budget should consider the complete project.

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That includes:

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• professional advice;

• approvals;

• technical design;

• construction;

• fixtures and finishes;

• temporary arrangements;

• landscaping;

• and a sensible contingency.

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The cheapest initial quotation is not always the best value.

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A well-planned project with clear drawings, realistic allowances and fewer surprises is usually much easier to manage.​​​

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Planning an extension and trying to understand the likely hidden extras?

 

Tell us about your property, the type of extension you are considering and the stage you have reached.

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Where appropriate, APN Extensions may introduce you to relevant independent professionals, consultants and service providers who can help you plan the next steps and obtain suitable quotations.

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You are under no obligation to appoint any provider introduced through APN.