What a feasibility review actually tells you about your home (using a real London example)

The most expensive decisions on a home project are made before anyone picks up a pencil. Not during the build, and not at the quote stage. Right at the start, when you are working out whether to extend, how far to go, and whether the numbers make sense at all. Get that wrong and no amount of good building work later will fix it.

Most people start the same way. Three builders come round, and three quotes come back that do not match. A friend who did their loft has strong opinions. Someone mentions planning permission and the whole thing suddenly feels risky. The problem is not a shortage of information. It is that none of it is about your house, your street and your budget. It is general advice applied to a specific building, and specific buildings do not read the advice.

A feasibility review fixes that at the point where fixing it is cheap. To show what one actually contains, rather than describe it in the abstract, here is a real review carried out by our practice, with the address and owner details removed.

The house, and the real problem

The property is a 1930s semi-detached chalet in the London Borough of Waltham Forest, held freehold, worth around £625,000. We have removed the address, the owners' names and anything that would identify the specific house; the figures and findings are the property's own. On paper it has four bedrooms. In practice it has one bathroom, and that bathroom is downstairs, serving three roof bedrooms above it. The reception rooms are cellular, cut off from one another in the way houses were built ninety years ago, and the kitchen is separated from a south-facing garden that the family barely uses as a result.

Nothing here is structurally wrong. The house is well built, on a quiet no-through road, with off-street parking and good schools nearby. The issue is arrangement, not size. That distinction matters, because it changes the whole approach. You do not need to bolt a large extension onto a house like this. You need to use what is already there properly. A builder pricing an extension would never tell you that, because it is not the question they were asked.

What the review put in front of them

A good feasibility review answers four questions in writing: what can this house become, what will planning allow, what will it realistically cost, and how far is it worth taking. Here is how each one played out.

The options, drawn for the actual house. Not mood boards, and not someone else's loft. Sketch plans of the real rooms, showing how the ground floor could open into a single kitchen, dining and living space facing the garden, and how the roof could be reorganised to give proper bedrooms, an upstairs bathroom and an ensuite. Two ground-floor variants were drawn so the owners had a genuine choice to settle between, rather than a single take-it-or-leave-it idea.

The real cost of each move. This is where quotes usually fall apart. Cheap quotes look cheap because they quietly leave things out: structural engineering, building regulations, party wall matters, contingency. Those items are not optional, they are just invisible until the build is underway and it is too late to say no. The review costed the scheme properly and room by room. Reconnecting the ground floor and garden came out around £111,000. Reorganising the roof and resolving the bathrooms, around £75,000. The full scheme, including bringing the tired fabric and services up to standard, landed near £186,000. Those numbers are indicative and firm up at detailed design, but they are honest starting figures, not a low number designed to win the job.

The planning position, from the record rather than a hunch. The house sits in no conservation area, and permitted development rights were broadly intact, which is a favourable start. More usefully, three houses on the same street had already been granted permission for near-identical work. That is not an opinion about what might be allowed. It is the council's own record of what has been approved a few doors away, which is about as strong a signal as planning ever gives you. The local authority here, Waltham Forest, decides around 84% of householder applications within the eight-week target and approves roughly two-thirds of minor applications, broadly in line with the regional average. The review rated each option for planning risk, and the headline was low.

The ceiling, so they knew when to stop. This is the part most people never get, and the part that protects you from the opposite mistake to under-building: over-investing. Improved comparables on the street pointed to a ceiling around £800,000. Set the roughly £186,000 of work against a £625,000 house heading toward £800,000, and there was comfortable headroom. Clear to proceed, but with a defined limit. Spend up to the ceiling and the money is sound. Spend past it and you are improving the house for the next owner, not for yourself.

The review also flagged where money could be saved by sequencing well. Much of the energy work, a heat pump eligible for a £7,500 government grant, insulation, new services, is far cheaper done while the structure is already open than retrofitted later. As the report put it, the biggest saving is rarely the grant. It is avoiding the cost of doing the same job twice.

The honesty that is built into it

Here is the uncomfortable part, and the reason a paid review beats a free consultation. Sometimes the honest answer is do not do this project, or the cheaper option gets you most of the benefit for a fraction of the cost. A free chat is unlikely to tell you that, because whoever gives it is hoping to win the bigger job. A review that stands on its own is free to say it.

That is the whole point of paying a few hundred pounds up front. You are not buying drawings. You are buying certainty about a decision that is about to cost you tens of thousands. If the review tells you to stop, you have spent £450 to avoid spending £45,000 badly. Knowing what not to do is the cheapest thing we sell.

Why now, on the wider numbers

The market context supports acting rather than waiting, though every household is different. London prices are still around 5% below their 2022 peak, interest rates have been easing, and build costs are forecast to keep rising, so waiting tends to cost more, not less. Moving is not the free alternative it feels like either: stamp duty, agent fees and removals in London can easily reach £50,000, and at the end of it you may still not have the house you actually wanted. Improving the one you have, up to its ceiling, often wins on the maths. A review is how you check whether that is true for your specific home before you commit.

How Stage Zero: Review works

This is exactly what our Stage Zero: Review delivers. It is a fixed-fee, design-led feasibility study of your property and your brief, produced before you commit to a full design appointment. Two versions: a desktop review at £450, produced from your plans, photographs and public records, and a site visit review at £600, which adds a home visit and a full measured survey so the options are drawn from accurate dimensions. Both give you a detailed written report within two weeks, plus a call to walk you through it.

If you go ahead with us to the next stage, the entire fee is credited against that work. If you do not, the report is yours to keep and use with any designer or builder you choose. There is no obligation beyond the review itself.

Studio MASS is a London residential design practice led by an architectural technologist with more than twenty years of experience and over 300 completed projects. We do not sell drawings, planning or extensions. We help London homeowners turn houses into long-term homes, starting with an honest answer about what yours could become.

If you have been sitting on a project, wondering whether it is worth it and what it would really cost, this is how you find out. Book your Stage Zero: Review, or see a sample report, at studio-mass.com/stage-zero.

Figures in this article are from a real anonymised review and are indicative and specific to that property. Costs firm up at detailed design. Planning outcomes depend on the individual application.