Ian's house before his extension

Nearly four months had zipped past since the idea of building a grandiose home extension was first conceived. My wife Ewa and I had made pretty good progress since then. Getting the full plans Building Regulations application submitted was a real milestone, allowing us to focus on finding suitable builders to turn plans into reality. But first there was one key decision that still needed to be made: whether to appoint a main contractor to undertake the whole thing, or opt instead to directly employ individual trades.

The safest way to run any sizeable building project is to draft detailed specifications, drawings and schedules of work for tendering to five or more main contractors and then have the winning bid wrapped up in a watertight contract. This way, risk is minimised, though the extra degree of security is reflected in the price.

But when the site happens to be your own home and you’re personally willing and able to project manage things on a day-to-day basis, adopting a more ‘hands on’ approach should allow you to retain greater quality control while reaping significant cost savings.

After some head-scratching we decided to break the project into two distinct phases to get the best of both worlds. We’d employ a main contractor to get the shell built to weathertight stage, then with the internal fitting-out work not being particularly time critical, money could be saved bringing in trades when required, leaving room for a smattering of DIY input. Without any pressing deadlines, but with a tight budget, it made sense to prioritise cost over speed.

Pricing the Job

With the property market buoyant, contractors are generally busy, which provides them with little incentive to price work competitively. Every second person you meet seems to have ‘got the builders in’, with the unquenchable demand for building services reflected in ballooning labour rates and shortages of some materials. However, we weren’t going to let such trifles tarnish our vision of a handsome new extension.

To pre-empt the risk of nasty surprises torpedoing the budget, I thought it worth taking time to update my draft costing before inviting builders to submit quotes. Trying to calculate the price of each course of brickwork or roof tiling in detail can be tricky in a rapidly rising market because published figures are often out of date. At the other end of the spectrum, the ‘big picture’ data tends to be more reliable, so overall build costs can be calculated to a surprising degree of accuracy by applying a price per m2.

Based on our final drawings, the ‘gross internal’ floor area for the extension came to 106m2 including all internal walls and both floors), equivalent to a ‘gross external’ area of 126m2 (which includes the main external walls). Surveyors normally work on gross external areas, but pretty much everyone else uses internal measurements. This is where online build costs calculators can be a real boon (like Homebuilding’s Extension Cost Calculator).

Ian's chosen build route had him pitching in with some DIY

Ian knew that to keep costs down he would have to pitch in during the build process.

Choosing the Build Route

Based on the desired level of build quality, the location of your property, the number of storeys and the gross internal floor area of the extension, it’s a simple task to calculate a ballpark total build cost. One useful feature is the option to select different procurement methods:

  • Main contractor
  • Main contractor and then self-managed subcontractors
  • Directly employed subbies
  • Mainly DIY plus some subbies.

Selecting a ‘good’ level of build quality (between ‘standard’ and ‘excellent’) and then picking the procurement option that had a mix of main contractor and self-managed subcontractors came up with a price of around £1,300/m2.

This gives a total build cost of nearly £140,000, although this level of quality doesn’t normally allow for ‘luxury items’ like underfloor heating and woodburning stoves. Picking the dearest ‘main contractor’ option added £10,000 to the overall costs, whereas the cheapest route (‘mainly DIY plus some subbies’) saved £15,000.

After a spot of tinkering to reflect our choice of main contractor for phase one only, the revised final figures came to £80,000 plus another £60,000 for the internal works.

When it comes to apportioning costs between labour and materials the precise split will depend on the quality of materials selected and the demand for labour, but, for an average project, splitting it roughly 50/50 shouldn’t be too far out.

With such an ambitiously sized extension we obviously needed to get the most bang for our buck. One way to do this is by taking responsibility for buying the materials, thereby benefiting from trade discounts at builders’ merchants. This also gives you considerably more control over quality and style.

The only problem is that unless you have a close relationship with your builders they may not relish sacrificing the profitable mark-ups charged on materials. They also may not want to risk poorly coordinated deliveries, which could result in delays on site. On the other hand, it saves them time if someone else does all the ordering, and it massively eases the cash flow situation, making the job a lot easier to slot into a busy schedule.

Hiring the Builder

We’d put out some early feelers to local builders, but many were booked up well beyond the year end. Enter GD Rawlings Building & Civils LLP, a family firm run by Godfrey Rawlings, a company I’d known since I’d featured photos of their brickwork in the Haynes’ Home Extension Manual I’d written. Rawlings subsequently carried out the structural work on our garage loft conversion back in 2007.

Ian chose GD Rawlings to build his extension project

GD Rawlings Building & Civils LLP was chosen to construct Ian’s monster 100m² extension

A quick call to building control revealed no immediate issues with the submitted drawings, so we could send these out for pricing, alongside the detailed specifications, with reasonable confidence.

The upshot was that two of the three priced specifications, submitted came in comfortably within our £40,000 guide price for phase one labour (the third one was poorly drafted and obviously a ‘fishing expedition’). The quoted prices allowed for all necessary plant, scaffolding and insurance, but not for loo hire and waste disposal, which would be up to me to arrange.

Godfrey told me he could fit the job in if he could get started on site in three months’ time. For clients and contractors alike, there’s a lot to be said for ‘better the devil you know’. Most builders can reel off shocking stories of non-payment, and knowing from experience that I was a ‘good payer’ surely worked in my favour.

For my part, I could be confident they’d do a decent job without pulling any annoying stunts. The only thing we needed now was the green light from building control.

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