Office to Residential Conversion Calculator

Class MA of the Town and Country Planning (General Permitted Development) (England) Order 2015 (as amended) is the statutory route for converting Use Class E buildings — which includes offices — to residential use without a full planning application. Eligibility is determined by a set of conditions that must all be satisfied before a prior approval application is submitted. Failure to meet any one of them means the development is not PD and a full planning application will be required.

The vacancy condition requires the building to have been vacant for at least three months immediately before the prior approval application date. This is assessed on a building-by-building basis; if a multi-floor office block has floors occupied by tenants on upward-only rent reviews, those floors cannot be included in the application. Developers frequently time acquisition and application to coincide with tenant departures, and evidence of vacancy (typically a statutory declaration supported by utility cut-off records, business rates relief notifications, or landlord correspondence) must accompany the prior approval submission.

The two-year use condition requires the building to have been in Use Class E (or a predecessor use that would now fall within Class E) for a continuous period of at least two years ending on the application date. Buildings that were recently converted from another use — or that have been vacant for longer than two years with no evidence of prior commercial use — may not satisfy this condition. Former residential buildings temporarily converted to offices, or sui generis uses such as casinos or nightclubs, do not qualify.

Office-to-residential conversion costs vary considerably depending on the age and specification of the building, the target residential product, the extent of reconfiguration required, and regional labour markets. As a broad benchmark, conversion costs in 2024–25 range from approximately £80–£120 per square foot for a basic open-plan conversion to £150–£220 per square foot for a full Cat A+ finish in a building requiring significant M&E (mechanical and electrical services) replacement, façade works, or structural intervention.

The most significant cost driver in office conversions is the M&E overhaul. Modern commercial offices run centralised HVAC, raised-access flooring, and shared WC cores that are entirely unsuitable for residential use. Stripping out and replacing these services — running new gas or air source heat pump infrastructure, individual flat ventilation, and separate metering — typically accounts for 25–35% of the total build cost. Where the building has asbestos (common in anything built before 1990), a full asbestos survey and remediation programme will add cost and programme time.

Fire compartmentation is another major cost variable. The Building Safety Act 2022 and associated changes to Approved Document B introduced significantly more onerous requirements for compartmentation in residential buildings, particularly those over 11 metres in height. For office buildings being converted above this threshold, a full fire strategy must be prepared by a competent fire engineer, and the cost of achieving the required compartmentation between flats (and between floors) in a building designed for open-plan commercial use should not be underestimated.

The residential unit mix — the combination of studio, one-bedroom, two-bedroom, and larger flats — is the primary lever through which developers maximise GDV from a fixed amount of floorspace. In an office-to-residential conversion, the unit mix is partly constrained by the structural grid of the building: offices designed around a 7.5–9 metre structural bay pattern will naturally lend themselves to one-bed and two-bed units, while narrower cellular office buildings may suit smaller studios or compact one-beds.

Studios and one-bedroom flats typically achieve the highest value per square foot in most UK markets, because the entry price point is accessible to a larger pool of buyers and investors. However, many LPAs have adopted minimum internal space standards — the nationally described space standard (NDSS) recommends a minimum of 37 sqm for a one-person studio — and some councils apply these as a condition on prior approval as well as on full planning. Below-standard units carry a real risk of mortgage refusal on exit, limiting the buyer pool to cash purchasers only.

The most important exit value comparison in any office conversion appraisal is the per-square-foot residential value versus the per-square-foot office value at the point of acquisition. In secondary provincial markets, this arbitrage can be significant — offices may trade at £80–£150/sqft capital value while comparable residential values are £200–£300/sqft. In prime London locations, the gap may be narrower, but the absolute values are higher. Sensitivity testing the GDV against a 5–10% downward movement in residential values is essential, particularly in markets where new-build residential supply from competing conversions is elevated.

The finance structure for an office conversion project depends principally on three variables: the total project cost (acquisition plus build), the strength and certainty of the planning position, and the developer's track record. For most projects, the preferred structure is a two-stage facility: a bridging loan to fund the acquisition and prior approval costs, transitioning to a development finance facility once prior approval is secured and construction is ready to commence.

Development finance for office conversions is typically available at 60–70% of total project costs (acquisition plus build), with interest rates currently in the range of 8–12% per annum depending on lender, LTV, and borrower profile. Lenders will appoint their own monitoring surveyor to certify drawdowns against a schedule of works, and will require a professional QS cost report before first drawdown. The monitoring surveyor's role is to protect the lender's security as well as to provide a framework for orderly drawdown, so developers should factor in the cost of this service (typically £3,000–£8,000 per project) alongside arrangement fees, legal costs, and valuation fees in the finance cost column of the appraisal.

Where a developer has an established relationship with a funder and strong prior approval, some lenders will underwrite the entire conversion under a heavy refurbishment bridging product at a higher LTV against GDV — typically 65–70% of the completed residential value. This removes the need for a transition between facilities but generally carries a higher all-in cost. The right approach depends on the programme — short, simple conversions suit a single bridging facility; complex, multi-phase projects or buildings requiring significant structural work typically warrant a full development finance facility with properly structured drawdown.