The first question almost everyone asks is the same one. How much will it cost. And the honest answer most homeowners hate to hear is, it depends. But that is not a dodge. The cost of an extension or conversion depends on real, knowable things, and a good company can walk you through every one of them.
The problem is that some companies hide behind that vagueness to win the job with a low number, then claw it back later. A trustworthy london design and build company does the opposite. It explains exactly what drives the price, so the figure you agree to is the figure you pay. Here is how the real cost actually gets worked out.
The Size and Type of Build Sets the Baseline
The obvious driver is what you are building. A single storey rear extension costs less than a double storey. A basement costs more than a loft. A simple dormer is cheaper than a full mansard.
This gives you a rough ballpark, usually quoted as a cost per square metre for your type of project. But that figure is only a starting point. Two extensions of the same size can cost very different amounts once the details come in.
A good company gives you the ballpark honestly, then explains what could push your project above or below it. The ones who quote a tempting low rate per square metre and stop there are not telling you the whole story.
The Ground and the Structure Move the Number
What is beneath your house matters a lot. Clay soil, nearby trees, a high water table, or shallow existing foundations all mean deeper, more expensive foundations.
The structural work matters too. A big open plan space needs more steel than a layout with walls left in place. Removing a load bearing wall costs more than working around it.
This is why a real quote comes after a proper look at your house, not from a phone call. A company that prices the structure and ground properly gives you a number that holds. One that guesses gives you a number that grows the moment the digger finds something.
The Finish Is Where Budgets Really Swing
Here is the part that surprises people most. Two identical extensions can differ massively in price purely because of the finish.
Kitchens range from a few thousand to many times that. Flooring, tiling, doors, windows, bathroom fittings, all of it spans a huge range. The shell of the extension is fairly fixed. The finish is where your choices send the budget up or down.
A good company helps you understand this so you can put money where it matters to you and save where it does not. They do not assume the most expensive option, and they do not hide cheap fittings in a low quote to make the number look good.
A Proper Quote Spells Everything Out
The single best protection you have against surprise costs is a detailed quote. Not a one line total, but a breakdown of what is included.
Foundations. Structure. Building regs. Roof. Windows and doors. Electrics and plumbing. Plastering. Making good. The more detail in the quote, the fewer surprises later. It also helps to look at a company’s real extension projects, since finished work shows you what your money actually buys far better than any number on a page does.
A vague quote is a warning sign. It usually means the gaps will be filled with extras once you are committed and it is too late to walk away.
The Cheapest Quote Is Rarely the Cheapest Job
This is the trap that catches the most people. Several quotes come in, one is clearly lower, and it feels like the sensible choice.
Then the extras start. Things that were not included. Change fees. Bits the cheap quote left out to look attractive. By the end, that low quote has often overtaken the higher ones you turned down.
The real cost of a job is not the number on the page at the start. It is the number you actually pay at the end. A slightly higher quote that includes everything is nearly always cheaper than a low one that keeps growing.
Honesty Upfront Tells You Who to Trust
In the end, the way a company talks about money before the job starts tells you how the whole project will run. A company that explains the costs clearly, flags the things that could change, and puts it all in writing is showing you how it works.
A company that is cagey about price, vague about what is included, or suspiciously cheap is showing you something too. On a project this big, that early honesty about cost is one of the best signs you have picked the right people.
Getting a clear breakdown from the start is not just about the money. It is about working with a company that respects you enough to tell you the truth before you commit.
