Building extensions: compare quotes and prices

What extension do you want to achieve?

A side extension can add rooms or an access route. Building above a garage or another existing part has different requirements. A linking structure between existing areas can also be a project in its own right. Describe the location and intended use first.

A heated living room, hobby area and unheated store are different tasks. State whether the new space will be used permanently and what functions are needed. A new building volume is not automatically a fitted room with every installation.

You can request design and construction together, work from existing documents or a clearly limited package. Existing design or site management should be considered in the offer. A simple internal alteration or surface renewal is not a new extension.

Clarify the site, existing building and intended use early

Free space on a plot does not prove that the proposed extension is possible there. Local requirements, permitted use, neighbours, infrastructure and the existing building affect design. These matters are clarified with the responsible professionals and local building authority.

Existing plans help but do not always replace measurement of the actual building. Dimensions, components and service routes in an older building may differ from its documents. An initial enquiry can be made without complete plans.

If an old extension, terrace or another area is to be removed, preparation, demolition and material handling need to be included in the agreed scope. A customer’s statement that an existing slab is stable does not replace assessment of its suitability for the new use.

Define the added area and room requirements

Give known length, width, height and the intended additional area. Explain whether this includes every new level or only the footprint of one level. Existing living area, new usable area and external landscaping are different quantities.

Describe the main use and particular preferences together: a new living area, an office, storage, a connecting door or additional entrance. Not every material or installation needs to be decided already.

For several extensions or different parts, known dimensions can be given in one answer. Missing technical details can remain open initially. Relevant measurements and connections need confirmation before final manufacture or construction.

Coordinate foundations, openings and building connections

The new part needs to suit the existing structure and ground. Loads, foundations, bearings and connections require professional assessment. A lightweight system or an existing concrete area does not automatically mean that no adaptation is needed.

Connection to an existing room may require changes to the facade, wall or roof. Clarify who creates openings, prepares the area and completes the finishes. The opening and extension need to be planned as a coordinated sequence.

Drainage, waterproofing, insulation and airtightness are particularly important at transitions. Levels and different component movements may affect detailed design. A finished wall next to the old house is not yet a fully connected building part.

Plan fit-out, services and continued occupation

Permanently used rooms need consideration of heat, summer comfort, light, sound and the requested fittings. Heating, electrical or plumbing work must be coordinated with the existing building and responsible contractors. A desired connection is not yet a confirmed technical possibility.

Timber, masonry or concrete and combined structures can be appropriate depending on the project. Prefabricated elements need early agreement on dimensions and connections. They guarantee neither the lowest overall price nor a short completion period for every project.

If the building remains in use, discuss protection, access and temporary restrictions. Agree weather protection during open phases and reinstatement of nearby surfaces. Storage, setup space and approach routes belong in installation planning.

Compare extension costs within the same boundaries

Added area influences price but does not explain it alone. Existing-building connections, foundations, geometry, use and fit-out can significantly change the work. A small extension involving several changes to the house is not simply priced in proportion to an independent area.

Check design, preparation, demolition, structure, roof, envelope, services and interior fit-out. Removal, protection, lifting equipment and reinstatement may also be needed. Fees or other parties’ work are included only when explicitly allowed for.

Compare the same area definition and final condition. A shell offer is not the overall price of a room ready for use. General square-metre figures without location, scope and VAT do not provide a reliable price basis. Unresolved existing conditions, estimates and later changes should remain identifiable.

Useful details for the first enquiry

A short description of the result you want helps the contractor prepare the necessary checks. Unresolved technical questions can be clarified with the professionals.

• Services. State design and construction, execution or a limited package.
• Position and use. Describe where the extension will be and its intended use.
• Area and dimensions. Give known added area and dimensions with a clear reference.
• Existing parts and extras. Mention available design, areas to retain and additional work.
• Location and timing. Give the location, relevant floor, site access and preferred period.

From the need for more space to a coordinated contract

Design considers the new and existing parts together. Important decisions precede ordering components and changing the existing building.

1. Describe the desired result. Describe the desired result, use, location and known situation.
2. Assess feasibility. Have the site, existing building, structure and necessary local checks assessed.
3. Define services. Define connections, fit-out and responsibilities in the specification.
4. Compare quotes. Compare quotes on the same basis and resolve open items.
5. Coordinate delivery. Agree preparation, protection, work sequence and handover with all parties.