A prefabricated home's advertised price is only the start of the budget

Find out what the advertised house contains

Ask whether the quoted area is usable internal area or built area, and what the supplied package includes. A shell, an assembled structure and a fully equipped dwelling are different offers. Request a specification covering structure, envelope, windows, internal finishes, installations, kitchen and any equipment shown in the images.

Use the same area definition and completion standard when comparing prefab suppliers or a conventional build. A cheaper figure can simply describe less building or fewer finished items. Confirm the division between factory work and work on site, together with who takes responsibility for the connections between them.

Establish whether the plot works before committing to the house

Check the proposed residential use and design with the relevant municipality and competent professionals. A prefabricated or modular description does not automatically exempt a dwelling from Spanish land-use and building checks. The actual plot, proposal and applicable procedure matter; do not rely on a general claim that a factory-made house needs no permission.

The site assessment should cover the necessary ground and level information, foundations, drainage and access. Confirm a workable transport and lifting route, rather than just access for a car. For utilities, establish available capacity, connection points and the work between them and the house. 'Water nearby' does not define the connection or its cost.

Official source: BOE · Ley de Suelo y Rehabilitación Urbana, artículo 11 · BOE · Ley 38/1999 de Ordenación de la Edificación

List everything outside the house package

Go through each part of the project and assign it to a named supplier or professional. If the house company supplies the module but another contractor prepares the foundations, the interface needs coordinated design, information and timing. An item being someone else's responsibility does not remove it from your total.

Include the professional and administrative work appropriate to the project, with the applicable authorisations, completion documentation and occupation procedure checked locally. Permanent residential buildings do not escape the relevant building framework because their components were prefabricated. Avoid adding an extra twice where the supplier has already included it.

Add the work outside the kit price

Consider an entirely hypothetical arithmetic example: a kit at €120,000, foundations at €18,000, transport and crane at €9,000, site and utility works at €14,000, and professional fees at €12,000. The subtotal is €173,000. It excludes land, taxes, local fees and contingency, which still need to be allowed for separately. These are invented amounts, not observed Spanish prices or a complete turnkey offer.

The kit is only part of that subtotal, and the amount needed to complete the project is still unresolved. Establish the tax status of each actual quotation and produce a final tax-inclusive budget. Use real site information and written prices to replace every assumption before making a commitment.

Compare offers on one shared project brief

Send suppliers the same plot information and requirements, including your intended finish and equipment standard. Ask for exclusions and provisional allowances to be stated clearly. If you will use finance, check the provider's requirements and the relationship between available funds and the proposed payment stages before agreeing them.

• Keep usable and built areas clearly labelled across all offers.
• Confirm who provides site preparation, foundations, delivery, lifting and assembly.
• Include the complete utility routes, drainage and any agreed external works.
• Identify professional services, local fees, taxes and completion requirements.
• Match payment stages to defined responsibilities and agreed evidence of progress.

Agree what handover means on this plot

The end of factory production is not the date the home becomes ready to occupy. Ask for a coordinated programme covering site readiness, delivery, assembly, connections, finishes and the relevant checks and documentation. Make clear who resolves a mismatch between site work and factory components and how changes are approved.

Before signing, agree what must be ready at handover: functioning installations, completed contracted finishes, relevant records and a process for outstanding defects. Check what still falls to you, including equipment or external work. A reliable total depends on everyone involved understanding what they must complete.

Official source: BOE · Ley 38/1999 de Ordenación de la Edificación