RICS recommends weighing the space needed, affordability, garden size, effect on neighbours and likely value when considering an extension. [1]
Decide what extra space is meant to do
Make a room-by-room brief before choosing a building type. For example, write “somewhere to work with a door that closes” rather than “a loft conversion”. Then test whether a different arrangement inside the existing house could satisfy that need.
Use a simple comparison sheet with the same headings for every option: usable rooms, access, garden, disruption, total funding and unresolved permissions. Do not give an option a positive score merely because a photograph or sketch looks appealing.
Test an extension and a loft on their own merits
An extension and a loft conversion involve different structural questions. Foundations, ground conditions and nearby structures matter for an extension; a loft needs an assessment of the roof, new floor loading and staircase arrangements. These are feasibility questions, not details to assume away until construction. [2]
Ask the designer to show furniture and circulation in the proposed rooms, and what happens to the existing accommodation. Check the result you would actually live with, not only the amount of new floor area.
Planning may also change the comparison. Permitted development rights are subject to limitations and conditions, and the householder rights do not simply apply to flats. Have the route checked for your particular property and proposal. [3]
Compare complete financial commitments
For building work, request a total that identifies construction, professional services, statutory charges, tax and any client purchases separately. RIBA explains that fees for professional services and payments to local authorities are separate cost items; ask whether a quotation includes them. [4]
For moving, prepare an equivalent schedule with your conveyancer and mortgage adviser: the price difference, transaction expenses, any tax due, borrowing changes and work needed at the replacement property. Leave uncertain amounts visibly marked rather than using them as zero.
A further advance is borrowing from your current mortgage lender, whereas remortgaging replaces the mortgage arrangement. Compare the total cost of borrowing, including charges and the repayment period, rather than just the additional monthly payment. [5] [6]
Do not make resale value the only test
Ask for a separate, property-specific view of likely sale value rather than treating the building budget as an automatic increase in value. RICS identifies value as one consideration, alongside suitability and affordability, when deciding whether to extend. [1]
Finish the comparison by answering: “Would we still choose this option for its usefulness if the resale uplift were less than hoped?” Your answer can help separate a home-improvement decision from a speculative investment decision.